Fix warnings in the PHPUnit workflow.

[costdev]

In the "Run PHPUnit tests" step of the PHPUnit workflow, there are a bunch of warnings related to test data files not being found. The paths are for themes in the test site used in the workflow.

While these warnings don't cause test failures, they do create unnecessary noise in the output.

[costdev]

N.B. Setting WP_RUN_CORE_TESTS to 0 in the config file doesn't have an effect on these warnings.

[afragen]

Where is this test theme added to the testing WordPress installation?

[kasparsd]

This doesn't seem to be happening anymore in latest test runs. I believe this can be closed.