Is your feature request related to a problem? Please describe.

Hi, I've recently tried to collaborate with the project and had some contact with test cases. I saw that there are a lot of tests that need to mock a sub-process call, and most of them accomplish this by creating a mock that calls a "cheaper" sub-process such as echo test.

But the project has a Stub for mocking the Popen class:
https://github.com/oppia/oppia/blob/6f2156f/scripts/scripts_test_utils.py#L28

As a suggestion: those scripts tests could be updated to use the stub, to run faster/cheaper tests.

Describe the solution (or solutions) you'd like

Replace occurrences of subprocess.Popen that actually call sub-processes in the scripts tests (at scripts/**/*_test.py) with the PopenStub Stub class.

Describe alternatives you've considered and rejected

No response

Additional context

No response