Refactoring Tests & Various Fixes for Symfony 6

[weaverryan]

Hi!

A big, kind of ugly change. Reasons:

• We needed a bit more flexibility to tweak the workflow of individual tests for different versions of Symfony
• Reduced duplication: the "fixtures" in the tests caused a lot of duplication, which has grown over time. This minimizes that quite a lot.

Fixes #1009, fixes #1005, fixes #984, fixes #900

Cheers!

[weaverryan]

This is ready! Though I don't seriously expect any detailed review :).

This is HUGE - sorry. It is 95% test refactoring and 3% fixing our test matrix to test a wider variety of Symfony versions and 2% fixing minor real bugs for different versions of Symfony that these enhanced matrix found.

The remaining failures for Symfony 6 are due to 10 tests where the package we're testing is not yet installable on Symfony 6. So, not much we can do there :). Those tests should start passing when those packages add Sf6 compat. If they continue failing, we'll then know we have a bug to track down.

Cheers!