Mono is not unloading collectible assemblies

[steveharter]

Placeholder for now until verified through CI.

The tests being added for #91290 that check if a collectible assembly is unloaded are not passing under Mono. I assume this should work.

Also, even if the Type-based caches that were added in the ActivatorUtilities.cs in the referenced PR are #ifdef'd out (like they are for NetFramework), the tests under Mono still fail suggesting the unload issue is general and that the usage of ConditionalWeakTable<Type, ConstructorInfoEx[]> in ActivatorUtilities.cs is not the reason the tests fail.

[jkotas]

Duplicate of #34072