Test parallelization stuff. (#31650)

This is the content analogue of https://github.com/space-wizards/RobustToolbox/pull/5412

Made Content.Tests run tests parallelized, which doesn't really seem to matter much.

I tried to make Content.IntegrationTest more aggressively parallelized but ran into https://github.com/dotnet/runtime/issues/107197 so we need to shelve that. Added a comment explaining as such.

Content.IntegrationTests/PoolManagerTestEventHandler.cs

@@ -1,7 +1,4 @@
-
-[assembly: Parallelizable(ParallelScope.Children)]
-
-namespace Content.IntegrationTests;
+namespace Content.IntegrationTests;
 
 [SetUpFixture]
 public sealed class PoolManagerTestEventHandler