Add unloadability testing tool (#22064)

Add unloadability testing tool (#22064)

* Add unloadability testing tool

This change adds the unloadability testing tool that can be used to run
coreclr tests or any other .NET core app inside of unloadable
AssemblyLoadContext, unload it after its execution completes and verify
that the unload succeeded. It has also various additional testing
options:
* memory leak testing
* running multiple iterations of the load/run/unload sequence
* optional breaking into debugger at various interesting stages (before
executing the test assembly, after executing it, on unload failure)
* delegated load when the AssemblyLoadContext that loads the test
assembly delegates loading of all the dependencies to another
AssemblyLoadContext