Investigate SDK tests running on .NET Framework to actually call msbuild.exe

In investigating https://github.com/dotnet/sdk/pull/49531, we found that we had tests that are running on the full framework leg that are simply calling dotnet msbuild.dll. This is just repeating testing we already have.

We believe that there are tests that call msbuild.exe but we don't have a good accounting.

TODO:

- Review the tests that compile into .netfx versions for the full framework leg
- Review which of these target msbuild.exe or not
- Change our core logic where if we were going to call dotnet msbuild.dll, just call msbuild.exe instead.
- Disable tests on full framework that can't be run by msbuild