Add more unit tests for the custom header feature

[bradyvercher]

The custom header functionality is largely untested. I introduced some tests in #38639, but they got overlooked while focusing on getting 4.7 out the door, so I'd like to reintroduce them for inclusion.

[bradyvercher]

39241.diff​ contains tests introduced in #38639 and they've at least had a cursory review here.

It doesn't look like the patch keeps binary files, so the video file will need to be ​downloaded from this repo and saved to tests/phpunit/data/images/video.mp4 to make the tests pass -- I wasn't sure if a new directory should be created for videos instead. According to the license, that file should be public domain:

To the extent possible under law, the author has waived all copyright and related or neighboring rights to this work.

[joemcgill]

Thanks for following up on these @bradyvercher. The tests look good and are passing here.

[obenland]

In 40825:

Themes: More unit tests for Custom Header

Custom Header functionality is largely untested. This adds tests for existing
behavior as well as changes introduced in [39227].

Props bradyvercher.
See #38639.
Fixes #39241.