Enable QuickGrid anchoring tests (#69221)

* Enable tests that were failing on introducing AnchorMode in QuickGrid and that are passing now.

* Move the stabilization to the shared WaitForRenderToSettle method.

* Re-enable as fixed by https://github.com/dotnet/aspnetcore/pull/67936.

* Unquarantine tests that have no record of failing in the last 30 days.

* Stabilize test setup that had problems in https://github.com/dotnet/aspnetcore/issues/68730#issuecomment-5634749318.

* Justification for unquarantine https://github.com/dotnet/aspnetcore/issues/68775#issuecomment-5634857078.

* Remove quarantined test attribute from VirtualizationTest

Removed quarantined test attribute from QuickGrid test. resolve https://github.com/dotnet/aspnetcore/pull/69221#discussion_r3989489124