Fix System.Runtime tests for single file runner

[MichalStrehovsky]

Allows us to test the documented single file behaviors.

Not adding a new PlatformDetection thing for this since we don't really have a way to detect single file that wouldn't end us up testing a tautology.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[agocke]

LGTM

[agocke]

I think this should always return <Unknown> now, FWIW

[jkotas]

this since we don't really have a way to detect single file that wouldn't end us up testing a tautology.

Can the single file test driver set something in the AppContext and the dynamic checks then biggy back on it?

I think building different test assemblies for single file testing is just going to cause pain.

[MichalStrehovsky]

I think building different test assemblies for single file testing is just going to cause pain.

Yeah, but this is what we're doing because the xunit runner won't work for single file.

If we ever bite the bullet and delete all of xunit except for the custom attributes as Assert.* methods (replacing the rest with a source generator that constructs a set of delegates to test methods and runs them), this will have to change.

If it starts looking like we'll need this detection in other tests, I'll redo this to use AppContext.