[RuntimeAsync] Make runtime async tests to build and pass locally.

[VSadov]

This change reorganizes moves individual tests into directories and adds missing parts to make the tests buildable.

Also fixes couple issues, that fell through the cracks when VM and JIT portions of the feature merged separately.

With this change async tests can be built and run locally together with other tests with a few steps that enable feature and the tests, and as long as local toolset compiler is patched with one that understands async2 syntax.

We will work on a more streamlined workflow. This is a step in that direction.

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[VSadov]

How async tests can now be built and run locally:

• patch the toolset compiler - basically do buildroslynnugets.bat per runtimelab instructions and copy tasks
  from the Microsoft.Net.Compilers.Toolset.5.0.0-async-16.nupkg (rename to .zip and open)
  to C:\Users\<user>\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-1.25220.1, or whatever version is in use by the build.
• uncomment add_compile_definitions(FEATURE_RUNTIME_ASYNC) - to enable the feature in the product.
• delete src\tests\async\Directory.Build.targets - to unblock async tests from building.

Then build/run tests as usual. Tests under async should build, run and pass.

We will work on a simpler workflow and enabling tests in CI (not in regular runs).
This is a step towards that.

[VSadov]

Thanks!!