[imorland]

Fixes: no issue.

Changes proposed in this pull request:
The testing guide's composer.json scripts now include test:unit:filter and test:integration:filter, with a short paragraph and examples showing how to run one test class or method instead of the whole suite. Core and the bundled extensions gained the same scripts in flarum/framework#5181, and new extensions get them from the CLI in flarum/cli#63.

Reviewers should focus on:

• The last sentence says CLI-generated extensions include the scripts. Merge this after feat(init): add test filter scripts to new extensions cli#63 is released.

Screenshot
N/A, text only.

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

Confirmed

• Frontend changes: tested on a local Flarum installation. (No code changes.)
• Frontend changes: tests are green (run yarn test in js/). (No code changes.)
• Frontend changes: tests have been added, or are not appropriate here. (No code changes.)
• Backend changes: tests are green (run composer test). (No code changes.)
• 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.

Required changes:

None.