Separate MultipleHives tests into regular FrameworkResolution and MultilevelLookup (Windows-only) test classes

[elinor-fung]

Follow-up for #116658 (comment)

Split the MultipleHives tests into a regular FrameworkResolution and a Windows-only MultilevelLookup class.

This also reduces the amount of tests we run against multi-level lookup:

• Reduce number of tested TFMs < 7.0 to just netcoreapp3.1 and net6.0
• Remove test for --fx-version - we have the non-multi-level lookup coverage and multi-level lookup does add a particularly interesting dimension

cc @dotnet/appmodel @AaronRobinsonMSFT

[Copilot]

Pull Request Overview

This PR splits the MultipleHives tests into two separate classes to clearly separate regular framework resolution tests from Windows-only multi-level lookup tests and reduces the total number of multi-level lookup scenarios. Key changes include:

• Removal of the MultipleHives tests.
• Introduction of a dedicated Windows-only MultilevelLookup test class.
• Refactoring of the shared test base and RunTest methods in FrameworkResolutionBase to support the new design.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
MultipleHives.cs	Removal of the old tests, now split into separated classes.
MultilevelLookup.cs	New Windows-only test class for multi-level lookup scenarios.
FrameworkResolutionBase.cs	Refactored RunTest method to streamline command construction.
FrameworkResolution.cs	New test class handling regular framework resolution logic.

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