`dotnet test` should respect platform

[Evangelink]

Is your feature request related to a problem? Please describe.

It's possible to use the platform configuration to change the context of what's being included to a build using Platforms: https://github.com/microsoft/vs-threading/blob/e700b506be5888ffb21df7313930151578ea6161/Microsoft.VisualStudio.Threading.slnx#L4

When passing the flag to dotnet test the build step will respect it but not the test step.

Describe the solution you'd like

It would be nice to have the test step respecting the argument too.

[Youssef1313]

This is related to #51398 (well, mostly duplicate).
We don't correctly parse sln/slnx files in dotnet test for MTP.

[nohwnd]

For vstest this works. The build is handled by msbuild and we just run the dlls that it spits out. When I provide /p:Platform=NonWindows the project in the example repo is skipped, when I mark more test projects with the exclusions they won't run.