Cleanup TFM handling for the new dotnet test for MTP

[Youssef1313]

Related to #45927

The logic around removing TFM from global properties and then making sure the global property is valid doesn't sound correct to me.

The user should be free to have their csproj with <TargetFramework>net8.0</TargetFrameworks> and then be able to do this:

dotnet restore -p:TargetFramework=net9.0
dotnet build -p:TargetFramework=net9.0 --no-restore
dotnet test -p:TargetFramework=net9.0 --no-build

The new logic will also do fewer re-evaluations, so it should be faster.

[Copilot]

Pull Request Overview

This PR cleans up the TargetFramework handling for dotnet test by ensuring that global properties are applied correctly and multi-targeting scenarios are properly handled.

• Removed manipulation of global properties to override the TargetFramework property prior to project load.
• Changed the control flow to handle scenarios where both TargetFramework and TargetFrameworks may be set, including iterating over multiple target frameworks.

Comments suppressed due to low confidence (1)

src/Cli/dotnet/commands/dotnet-test/SolutionAndProjectUtility.cs:129

• Clearing 'TargetFrameworks' on each iteration may discard useful multi-targeting information needed by subsequent evaluations. Consider whether this property should be cleared before the loop or retained for later use.

project.SetProperty(ProjectProperties.TargetFrameworks, string.Empty);

[Evangelink]

@baronfel @rainersigwald could we have a confirmation here?

[baronfel]

This seems reasonable, however I would strongly disagree with users passing -p:TargetFramework=XXX on the command line. We have an established flag for doing that (-f/--framework) and we should not encourage users to do things in nonstandard ways. That may mean that you need to make sure to handle the SDK's concept of 'forwarded options' in this code path (I don't know the specifics of if/how you're doing that now) but that's something you just have to do in order to keep to our established UX interaction patterns.

[Youssef1313]

I think -f is already handled and is forwarded correctly. We end up setting the passed value as a global property either way.

[Youssef1313]

/azp run

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