Test-only changes for recipe projects needs real test files not symlinks

Recipe project PHPUnit test files need to be "real" and not symlinked from the top-level build folder. We already do this for the phpunit jobs automatically, but it is not done in the test-only changes job.

I found this while working on the GTD d11-recipe common config file.

Test-only changes failure https://git.drupalcode.org/project/gitlab_templates_downstream/-/jobs/9121441

This can be fixed by adding in the same copy code, as demonstrated in this temporary change - the phpunit tests run https://git.drupalcode.org/project/gitlab_templates_downstream/-/jobs/9122299

The existing lines in the PHPUnit script can be moved out into a referenced section, and then used in both the PHPUnit and Test-only Changes jobs.