test: add frontend tests

[SychO9]

Fixes #3990

Changes proposed in this pull request:
Adds some minimal frontend tests to more easily catch introduced issues. These are kept minimal so as not to hamper future development work with having to constantly update tests.

• Gonna need to add the Jest infra to the CLI + document how one can write a frontend test in Flarum, so info on that will be included in a follow up docs PR.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)