[.NET] Run the BCL tests

[rolfbjarne]

Currently we're only running our own tests on .NET (introspection, monotouch-test, linker tests, etc), not the BCL tests.

We should look into if we want to do this, and if it's even possible.

If we decide to do it:

• Get the BCL test assemblies for .NET. As far as I know they're not published anywhere, so this is probably the most complicated step.
• Add support to xharness for running them with .NET.
• Run the tests on CI.

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Time estimate: figuring out where to get the BCL test assemblies and getting those is probably the biggest task, and it's hard to estimate since it's unknown how much work that would be, but a wild guess would be ~2 weeks. Adding support to xharness once that's done is ~1 week, so 3 weeks total.

[rolfbjarne]

CC @akoeplinger @jonpryor this is our issue related to getting the BCL tests running in a .NET 6 context

[rolfbjarne]

This will not be done in the .NET 7 time frame due to time constraints, so moving to .NET 8.

[rolfbjarne]

We've decided to not to this for the time being. This can be re-evaluated if we run into regressions we'd catch if we were to run the BCL tests.