Running tests as a multisite fails to submit testing report

[desrosj]

Describe the bug
When the test runner is configured to run the unit test suite as a multisite, the results will not be submitted successfully because no junit.xml file is produced.

I've opened a ticket upstream in Core Trac, but I'm creating this to follow up once that's addressed, and to consider any other functional differences when running the tests as a multisite install.

To Reproduce
Steps to reproduce the behavior:

1. Prepare and run the test runner with WPT_FLAVOR=1.
2. Run the reporting script.
3. Observe a failure.
4. Inspect the tests/phpunit/build/logs directory and see that there is no junit.xml file.

[desrosj]

This was merged in r60781, but needs some testing to confirm.