Make all jobs report artifacts use source relative paths.

Problem/Motivation

Similar to #3397162: Tweak PHPStan config so paths are always correct and baseline is more usable →

GitLab expects all report paths to be relative to the build, how very many of our reports include the full path

For example:

https://git.drupalcode.org/project/keycdn/-/pipelines/162883/test_report...

Fillename: /builds/project/keycdn/tests/src/FunctionalJavascript/ExampleTest.php

Clicking on the path returns:

"...keycdn/tests/src/FunctionalJavascript/ExampleTest.php" did not exist on "01b04a7a410c2eeed34d2a5495cf73ea8d6ba8b4"

The incorrect base path makes it harder/slower to review code.

Steps to reproduce

Cause each test that generates assets to generate a report.

Proposed resolution

Ensure all report paths are relative to the source root.

Remaining tasks

Determine which tests generate reports that are not relative
to the source root.
Modify each test to cleanup the assets.

User interface changes

API changes

Data model changes