Add support for .slnx file exclusion in BaselineTest (#62767)

This update introduces a new condition to the `relativePath` checks in `BaselineTest.cs`. The code now excludes files that end with the `.slnx` extension, enhancing the filtering of project-related file types and directories during processing.

Author: marcominerva

src/ProjectTemplates/test/Templates.Tests/BaselineTest.cs

@@ -92,6 +92,7 @@ public async Task Template_Produces_The_Right_Set_Of_FilesAsync(string arguments
                 relativePath.EndsWith(".fsproj", StringComparison.Ordinal) ||
                 relativePath.EndsWith(".props", StringComparison.Ordinal) ||
                 relativePath.EndsWith(".sln", StringComparison.Ordinal) ||
+                relativePath.EndsWith(".slnx", StringComparison.Ordinal) ||
                 relativePath.EndsWith(".targets", StringComparison.Ordinal) ||
                 relativePath.StartsWith("bin/", StringComparison.Ordinal) ||
                 relativePath.StartsWith("obj/", StringComparison.Ordinal) ||