Test fixes to react to the slnx solution file change

Author: mkArtakMSFT

Diff for: src/DefaultBuilder/test/Microsoft.AspNetCore.FunctionalTests/WebHostFunctionalTests.cs

@@ -237,7 +237,7 @@ private static string GetTestSitesPath()
         var directoryInfo = new DirectoryInfo(applicationBasePath);
         do
         {
-            var solutionFileInfo = new FileInfo(Path.Combine(directoryInfo.FullName, "DefaultBuilder.slnf"));
+            var solutionFileInfo = new FileInfo(Path.Combine(directoryInfo.FullName, "DefaultBuilder.slnx"));
             if (solutionFileInfo.Exists)
             {
                 return Path.GetFullPath(Path.Combine(directoryInfo.FullName, "testassets"));

Diff for: src/Servers/test/FunctionalTests/Helpers.cs

@@ -16,7 +16,7 @@ public static string GetApplicationPath()
         var directoryInfo = new DirectoryInfo(applicationBasePath);
         do
         {
-            var solutionFileInfo = new FileInfo(Path.Combine(directoryInfo.FullName, "FunctionalTests.slnf"));
+            var solutionFileInfo = new FileInfo(Path.Combine(directoryInfo.FullName, "FunctionalTests.slnx"));
             if (solutionFileInfo.Exists)
             {
                 return Path.GetFullPath(Path.Combine(directoryInfo.FullName, "..", "..", "testassets", "ServerComparison.TestSites"));