_MTPBuild should only build if MTP project

[Youssef1313]

Fixes #50307

FYI @dsplaisted @baronfel

The scenario is:

• Solution with two projects
• First project is a test project targeting netX.0
• Second project is a class lib targeting any other different TFM (e.g, netstandard2.0), and it's not test project.
• First project references the second project.
• User does dotnet test --framework netX.0
  • Previous behavior, we will attempt to build the classlib with netX.0. But as netX.0 wasn't part of restore, this fails.
  • New behavior, we will attempt to only build the test projects. This is aligned with the existing behavior for VSTest, and is potentially a performance improvement for scenarios where a solution has some projects that are not present in the dependency graph of test projects, as we will not need to build such projects altogether.

[Evangelink]

I think the original behavior is correct. If you create a class library project targeting netstandard2.0 and a console app targeting net8.0 and referencing the classlib and you do dotnet build --framework net8.0 you will have the error:

The --framework is a build argument so it's flown down and causes a failure. If we want to introduce a new argument for the test that is applied only to test project we could.

@baronfel What's your view on the behavior of SDK? I mean is a higher command that includes lower commands should respect the lower command behavior or change it?

[Youssef1313]

Discussed offline with @Evangelink and we are now both aligned that the change in this PR is okay. I'd still would love hear @baronfel feedback :)