[random test failure] Try to un-skip and fix MediaSourceFileTest::testMediaFileSource in context of [#3353085]

Problem/Motivation

In #3350972: [random test failure] Drupal\Tests\layout_builder\FunctionalJavascript\LayoutBuilderUiTest::testReloadWithNoSections() @nod_ introduced a "fix" for problems with the correct opening of an off-canvas dialog which potentially could fix all JS random test failures for tests using the drupal.dialog.off_canvas somehow.

The basic test is:

1. Unskip \Drupal\Tests\media\FunctionalJavascript\MediaSourceFileTest::testMediaFileSource.
2. Run _only_ now unskipped \Drupal\Tests\media\FunctionalJavascript\MediaSourceFileTest::testMediaFileSource a lot of times (usually we go for 1500x) as-is.
3. Run _only_ now unskipped \Drupal\Tests\media\FunctionalJavascript\MediaSourceFileTest::testMediaFileSource a lot of times (usually we go for 1500x) _without_ the changes in #3350972: [random test failure] Drupal\Tests\layout_builder\FunctionalJavascript\LayoutBuilderUiTest::testReloadWithNoSections().
4. If 2., passes and 3. doesn't, we can safely turn the test back on again.

Per @xjm in #3353085-4: [meta] Determine impact of [#3350972] fix in off-canvas.js on currently disabled FunctionalJavascript tests we now need to go for a 5000-8000 times run for both patches.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet