Provide a way to separate target and runtime framework with dotnet test

[MichalPetryka]

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

Testing a library that provides both netstandard and net target frameworks proves to be difficult, since test projects can't target netstandard. In case of for example netstadard2.1 and net6.0, this forces users to dual target the tests to netcoreapp3.1 and net6.0, the first of which soon goes out of support and forces the developers to install more SDK versions than necessary.

Describe the solution you'd like

Adding separate --target-framework and --runtime-framework parameters would make this possible, the --framework parameter would then set both to the same value.

Additional context

This'd probably require test frameworks to add netstandard versions of the packages and would require IDE support to be added.