Fix slnf test assets to use \\

[Youssef1313]

Closes #51521

I think the PR might fail currently, but hopefully will pass after #51411 is merged.

[Copilot]

Pull Request Overview

This PR fixes the path separator convention in solution filter (.slnf) test assets to use double backslashes (\\) instead of forward slashes. The change aligns these test assets with the JSON format standard and the existing convention used in all other .slnf files in the repository, where backslashes must be escaped in JSON strings.

Key Changes:

• Updated path separators from / to \\ in .slnf test asset files to match the repository-wide convention

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/TestAssets/TestProjects/MultiTestProjectSolutionWithTests/TestProjects.slnf	Changed project path separator from / to \\
test/TestAssets/TestProjects/MultiTestProjectSolutionWithTests/SolutionFilter/OtherTestProjects.slnf	Changed both solution path and project path separators from / to \\

[Youssef1313]

[xUnit.net 00:02:14.69]     Microsoft.DotNet.Cli.Test.Tests.GivenDotnetTestBuildsAndRunsTestsWithDifferentOptions.RunWithSolutionFilterPathInOtherDirectory_ShouldReturnExitCodeGenericFailure(configuration: "Debug") [OUTPUT] ❌Could not find solution or directory `/private/tmp/helix/working/AC330920/t/dotnetSdkTests.A6sEr5wd/a7b04b23-5863---FEB8A6C7/SolutionFilter/..\MultiTestProjectSolutionWithTests.sln`.
< /tmp/helix/working/AC330920/p/d/dotnet test --configuration Debug exited with 1 in 454 ms.

Let's see after #51411.