[2.x] fix(testing): reorder boot sequence in OverrideExtensionManagerForTests

[DavideIadeluca]

Fixes #4600
Reorder the boot sequence in OverrideExtensionManagerForTests to fix locale translations never loading in integration tests as described in #4600. LocaleManager was being resolved during onEnable() before extend() had registered the locale resolving() callbacks. This fix runs extend() first (with booted=true), then the enable() loop (with booted=false so migrations still run).

Changes proposed in this pull request:

Reviewers should focus on:

• Whether the booted toggle could cause side effects which the testing suite of the framework itself doesn't catch

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)

[DavideIadeluca]

Let's revisit this after 2.0 stable has been released. Fixing #4600 is technically a breaking change in the testing suite as e.g. 2nd or 3rd party vendors might already have assertions in place which assume that language packs are not working