feat(init): add test filter scripts to new extensions

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
New extensions get test:unit:filter and test:integration:filter composer scripts alongside test:unit and test:integration, so a developer can run one test class with composer test:integration:filter -- ClassName instead of the whole suite. The same scripts were added to core and every bundled extension in flarum/framework#5181. They belong to the backend testing module, so enabling it on an existing extension adds them too, and an extension without it gets neither.

Reviewers should focus on:

• Two new init tests: the scripts are present with backend testing, and no test script is generated without it. The first failed before the change.

Screenshot
N/A

Necessity

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

Confirmed

• Frontend changes: tested on a local Flarum installation. (N/A, CLI.)
• Frontend changes: tests are green (run yarn test in js/). (yarn test: 297 passed.)
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test). (N/A)
• Backend changes: tests have been added, or are not appropriate here. (N/A)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.