Context
What is the issue that we're seeing that is motivating this decision or change?
Per T365976#9933458, when parallel execution of PHPUnit tests are enabled, the test suites run should be the same as those run when PHPUnit parallel execution is disabled. It would be nice to have a safety net, though, in case something is wrong with the code that generates test suites for running tests in parallel.
Proposal
What is the change that we're proposing and/or doing?
If we have a daily job that runs PHPUnit tests serially (instead of parallel), we'll have a safety net to ensure that our tests run as intended and that we don't ship code that somehow passes in a parallel execution context but fails when the tests are run serially.
Consequences
What becomes easier or more difficult to do because of this change?
- We'll have more confidence in rolling out parallel tests in CI
- We'll need to have a process for monitoring the serial test run, as it indicates a potentially serious breakage