Overview

1. This PR fixes Avoid Redundant Builds and Installation in CI E2E tests #16028 and fixes Refactor GitHub Actions Workflows to Reduce Redundant Code #12647.
2. This PR does the following:
   • Enables caching of pip dependencies to reduce our susceptibility to network problems
   • Runs each E2E test as a separate job. This makes it easier to find E2E test runs of interest because one no longer has to keep track of which suite is in which workflow's job.
   • Uses a strategy matrix to auto-generate the job for each E2E test. This reduces duplicate code across our E2E test workflow files.
   • Before all the E2E jobs are started, download dependencies and compile webpack. Then make these prepared files available for each E2E test run, reducing overall runtime by avoiding redundant downloads and webpack compilation. Avoiding redundant downloads also reduces our susceptibility to network problems.

Essential Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The linter/Karma presubmit checks have passed locally on your machine.
• "Allow edits from maintainers" is checked. (See here for instructions on how to enable it.)
  • This lets reviewers restart your CircleCI tests for you.
• The PR is made from a branch that's not called "develop".

Work remaining:

• Confirm that we can pass webpack files between jobs as an artifact. Since building webpack is the bulk of our installation (~20 mins), reusing these is the most important step.
• Confirm that dependencies in oppia_tools are correctly being persisted to the second job. Installing dependencies is the next most time-consuming step (~4 mins), and since we cache pip packages (new in this PR) and node modules, most of the savings will likely come from oppia_tools. To check this, make sure that the installing dependencies step in the second job is fast.
• Use a matrix to auto-generate jobs for each e2e suite from a single job section in the workflow file. This will cut down on code duplication and let us run every e2e suite as its own job.

Proof that changes are correct

Proof that pip caching works

In the CI runs on this PR (example) we load pip-tools from the cache:

Collecting pip-tools==6.6.2
  Using cached pip_tools-6.6.2-py3-none-any.whl (47 kB)

In contrast, in a CI run from develop (example), we download pip-tools:

Collecting pip-tools==6.6.2
  Downloading pip_tools-6.6.2-py3-none-any.whl (47 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 47.7/47.7 kB 2.0 MB/s eta 0:00:00

Proof that third-party dependencies transfer

Notice that in this CI run the install-oppia-dependencies step takes only ~2 mins instead of the usual ~4 mins. This speed-up is because third-party dependencies like the google cloud SDK don't have to be downloaded--they were loaded from the zip file.

Proof that upload/download is worthwhile

Comparing this new CI run to an old run we can see the impact of these changes on overall runtime. Note that we previously had 17 E2E test workflows, and we have 43 E2E test suites.

Therefore even though we spin up more jobs with this approach and transfering build files as an artifact takes time, the benefits from only having to download dependencies and build webpack once mean that this transition will save us time overall.

Proof that all tests work

See CI runs: https://github.com/U8NWXD/oppia/actions/runs/3545519837

PR Pointers

• Make sure to follow the instructions for making a code change.
• If you need a review or an answer to a question, and don't have permissions to assign people, leave a comment like the following: "{{Question/comment}} @{{reviewer_username}} PTAL". Oppiabot will help assign that person for you.
• For what code owners will expect, see the Code Owner's wiki page.
• Make sure your PR follows conventions in the style guide, otherwise this will lead to review delays.
• Never force push. If you do, your PR will be closed.
• Some of the e2e tests are flaky, and can fail for reasons unrelated to your PR. We are working on fixing this, but in the meantime, if you need to restart the tests, please check the "If your build fails" wiki page.