This pull request focuses on refactoring and improving the handling of solution and project files in the CLI, as well as enhancing the build process. The most important changes include the introduction of new methods for parsing solution filter files, simplifying the MSBuildHandler class, and updating constants to support solution filter files.

Refactoring and Enhancements:

• src/Cli/dotnet/SlnFileFactory.cs: Introduced new methods GetSolutionPathFromFilteredSolutionFile and ParseSolutionFilterFile to handle solution filter files, and refactored CreateFromFilteredSolutionFile to use these methods. [1] [2] [3] [4]

Simplification of MSBuildHandler:

• src/Cli/dotnet/commands/dotnet-test/MSBuildHandler.cs: Removed redundant methods and simplified the build process by delegating solution and project handling to MSBuildUtility. Also, removed the degree of parallelism parameter and the validation of build path options. [1] [2] [3] [4] [5]

Support for Solution Filter Files:

• src/Cli/dotnet/commands/dotnet-test/CliConstants.cs: Added support for .slnf files by updating the SolutionExtensions array and introducing SolutionFilterExtensionPattern.
• Solution filter files are now supported using --solution, e.g. dotnet test --solution "TestProjects.slnf".
• If we run dotnet test and there exists only one .slnf file in the directory, we will run projects inside this .slnf file. But if we have multiple .slnf files, we will throw an error to specify the solution file. If we have 1 solution file (.sln/.slnx) and multiple .slnf files, we run only the solution file.

Relates to #45927