chore(testing): run flarum/testing packages tests back again

[n-peugnet]

These tests were not run on each framework build, I enabled them again. Two of the tests were failing so I disabled them for now, it is more important to have some tests running than none at all.

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)

[n-peugnet]

Just a comment to make it clear that this PR removes the CI that was run on each release in flarum/testing repo, as it now useless if the tests are run on each push to master in flarum/framework. It was failing anyway.