Fix a unit-test that missed to react to the .slnx change

Author: mkArtakMSFT

src/Testing/test/TestPathUtilitiesTest.cs

@@ -16,7 +16,7 @@ public class TestPathUtilitiesTest
     public void GetSolutionRootDirectory_Throws_IfNotFound()
     {
         var exception = Assert.Throws<Exception>(() => TestPathUtilities.GetSolutionRootDirectory("NotTesting"));
-        Assert.Equal($"Solution file NotTesting.slnf could not be found in {AppContext.BaseDirectory} or its parent directories.", exception.Message);
+        Assert.Equal($"Solution file NotTesting.slnx could not be found in {AppContext.BaseDirectory} or its parent directories.", exception.Message);
     }
 #pragma warning restore 0618
 }