[PP-2] Test Seven's changes to Media Library

Problem/Motivation

In #3082690: Mark Media Library as a stable core module → and related issues, a lot of Media Library's front-end code was moved into the Seven theme. Initially, it was all tested by MediaLibrarySevenTheme, which extended MediaLibraryTest and duplicated all of its coverage with Seven as the active theme.

However, because MediaLibraryTest was an unreliable, nightmarish monstrosity of a test, it was decided to remove MediaLibrarySevenTest from #3082690: Mark Media Library as a stable core module → and, once MediaLibraryTest had been refactored and stabilized, restore the test coverage for Seven's modifications.

Proposed resolution

Add a new MediaLibrarySevenTest which runs through all parts of the media library that Seven touches, and asserts that the changes are made and do not break anything.

Remaining tasks

Add the test once #3087227: [META] Split up and refactor MediaLibraryTest → , and ideally 🐛 [random test failure] Random failures building media library form after uploading image (WidgetUploadTest) Closed: cannot reproduce are addressed.

User interface changes

None.

API changes

None; Drupal's tests are not APIs.

Data model changes

None.

Release notes snippet

None needed.