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/PoolManager.cs

@@ -23,8 +23,6 @@ using Robust.Shared.Prototypes;
 using Robust.Shared.Timing;
 using Robust.UnitTesting;
 
-[assembly: LevelOfParallelism(3)]
-
 namespace Content.IntegrationTests;
 
 /// <summary>