Is your feature request related to a problem? Please describe.
#4983 introduced linter wrappers for multiple languages (to replace the existing shell scripts). These require unit tests for behavior regression verification.

Describe the solution you'd like
Each of the new, untested classes in the PR need tests to cover their main behaviors. Below are the new classes and minimum tests to add (more are likely needed, these are just a first pass):

• Add tests for Buf (suggest using the command locally to get an idea on the sort of output that the class will encounter)
  1. testMain_noArguments_failsWithError
  2. testMain_twoArguments_failsWithError
  3. testMain_nonExistentRepoRoot_failsWithError
  4. testMain_existingRepoRoot_noProtoBufs_passes
  5. testMain_existingRepoRoot_withProtoBufs_noLintErrors_passes
  6. testMain_existingRepoRoot_withProtoBufs_withLintErrors_failsWithError
  7. testRunBuf_nonExistentRepoRoot_throwsException
  8. testRunBuf_noProtoBufs_outputsPass
  9. testRunBuf_withProtoBufs_outsideSupportedDirs_noLintErrors_outputsPass
  10. testRunBuf_withProtoBufs_outsideSupportedDirs_withLintErrors_outputsPass
  11. testRunBuf_withProtoBufs_withinSupportedDirs_noLintErrors_outputsPass
  12. testRunBuf_withProtoBufs_withinSupportedDirs_withLintErrors_throwsException
• Add tests for Buildifier (suggest using the command locally to get an idea on the sort of output that the class will encounter)
  1. testMain_noArguments_failsWithError
  2. testMain_oneArgument_failsWithError
  3. testMain_twoArguments_invalidMode_failsWithError
  4. testMain_threeArguments_failsWithError
  5. testMain_nonExistentRepoRoot_failsWithError
  6. testMain_check_existingRepoRoot_noBazelFiles_passes
  7. testMain_check_existingRepoRoot_withBazelFiles_noLintErrors_passes
  8. testMain_check_existingRepoRoot_withBazelFiles_withLintErrors_failsWithError
  9. testMain_fix_existingRepoRoot_withBazelFiles_noLintErrors_passes
  10. testMain_fix_existingRepoRoot_withBazelFiles_withLintErrors_passesAndFixesErrors
  11. testRunBuildifier_check_nonExistentRepoRoot_throwsException
  12. testRunBuildifier_check_noBazelFiles_outputsPass
  13. testRunBuildifier_check_withBazelFiles_noLintErrors_outputsPass
  14. testRunBuildifier_check_withBazelFiles_withLintErrors_throwsException
  15. testRunBuildifier_fix_nonExistentRepoRoot_throwsException
  16. testRunBuildifier_fix_noBazelFiles_outputsPass
  17. testRunBuildifier_fix_withBazelFiles_noLintErrors_outputsPassNothingToFix
  18. testRunBuildifier_fix_withBazelFiles_withFixableLintErrors_ouputsPassAndFixesFiles
  19. testRunBuildifier_fix_withBazelFiles_withUnfixableLintErrors_throwsExceptionAndFixesSomeIssues
• Add tests for Checkstyle (suggest using the command locally to get an idea on the sort of output that the class will encounter)
  1. testMain_noArguments_failsWithError
  2. testMain_twoArguments_failsWithError
  3. testMain_nonExistentRepoRoot_failsWithError
  4. testMain_existingRepoRoot_noJavaFiles_passes
  5. testMain_existingRepoRoot_withJavaFiles_noLintErrors_passes
  6. testMain_existingRepoRoot_withJavaFiles_withLintErrors_failsWithError
  7. testRunCheckstyle_nonExistentRepoRoot_throwsException
  8. testRunCheckstyle_noJavaFiles_outputsPass
  9. testRunCheckstyle_withJavaFiles_outsideSupportedDirs_noLintErrors_outputsPass
  10. testRunCheckstyle_withJavaFiles_outsideSupportedDirs_withLintErrors_outputsPass
  11. testRunCheckstyle_withJavaFiles_withinSupportedDirs_noLintErrors_outputsPass
  12. testRunCheckstyle_withJavaFiles_withinSupportedDirs_withLintErrors_throwsException
• Add tests for Buildifier (suggest using the command locally to get an idea on the sort of output that the class will encounter)
  1. testMain_noArguments_failsWithError
  2. testMain_oneArgument_failsWithError
  3. testMain_twoArguments_invalidMode_failsWithError
  4. testMain_fiveArguments_failsWithError
  5. testMain_nonExistentRepoRoot_failsWithError
  6. testMain_check_existingRepoRoot_noKotlinFiles_passes
  7. testMain_check_existingRepoRoot_withKotlinFiles_noLintErrors_passes
  8. testMain_check_existingRepoRoot_withKotlinFiles_withLintErrors_failsWithError
  9. testMain_fix_existingRepoRoot_withKotlinFiles_noLintErrors_passes
  10. testMain_fix_existingRepoRoot_withKotlinFiles_withLintErrors_passesAndFixesErrors
  11. testRunKtlint_check_nonExistentRepoRoot_throwsException
  12. testRunKtlint_check_noKotlinFiles_outputsPass
  13. testRunKtlint_check_withKotlinFiles_noLintErrors_outputsPass
  14. testRunKtlint_check_withKotlinFiles_withLintErrors_throwsException
  15. testRunKtlint_fix_nonExistentRepoRoot_throwsException
  16. testRunKtlint_fix_noKotlinFiles_outputsPass
  17. testRunKtlint_fix_withKotlinFiles_noLintErrors_outputsPassNothingToFix
  18. testRunKtlint_fix_withKotlinFiles_withFixableLintErrors_ouputsPassAndFixesFiles
  19. testRunKtlint_fix_withKotlinFiles_withUnfixableLintErrors_throwsExceptionAndFixesSomeIssues
  20. testRunKtlint_generate_noInputScript_throwsException
  21. testRunKtlint_generate_noOutputScript_throwsException
  22. testRunKtlint_generate_nonExistentInputScript_throwsException
  23. testRunKtlint_generate_invalidShellScript_outputsEmptyFile
  24. testRunKtlint_generate_validShellScript_extractsJarFromFile

Describe alternatives you've considered
N/A -- All utilities require tests.

Additional context
Each of the utilities above already have existing placeholder test suites that are ready for new tests to be added, and should already build, run, and pass.

All other script & script utility tests are good to reference when building these. Note that these tests can only be run with Bazel and syntax highlighting/auto completion won't work in Android Studio unless using the Bazel plugin.

Note also that these tests often require creating real test data (such as APKs) to verify their functionality. This should be done without needing to permanently check in any binary or test files to the repository (prefer generating the files at runtime during the test run, instead), aside from changing the existing test suite files.