chore: add test:unit:filter and test:integration:filter scripts

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
Running one test class meant either the whole suite (core's integration suite is over 1,000 tests and takes several minutes) or calling PHPUnit by hand with --filter. This adds test:unit:filter and test:integration:filter to core and to every bundled extension with tests, used as composer test:integration:filter -- ClassName (or a method name, or 'ClassA|ClassB'). messages, mentions and suspend already had the integration one; it is moved next to test:integration, so every package lists the scripts in the same order.

New extensions get the same scripts from the CLI in flarum/cli#63, and the testing guide documents them in flarum/docs#664.

Reviewers should focus on:

• Scripts only: 19 composer.json files, nothing else changed in them.

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, dev scripts.)
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (No frontend changes.)
• Frontend changes: tests are green (run yarn test in js/). (No frontend changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No frontend changes.)
• Backend changes: tests are green (run composer test). (No code changes; ran test:unit:filter in core and test:integration:filter in sticky.)
• Backend changes: tests have been added, or are not appropriate here. (No code changes.)
• 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.