Use solution-relative contentRoot path, when the path frmo metadata either is not available or doesn't exist.

Author: mkArtakMSFT

src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs

@@ -249,7 +249,7 @@ private void SetContentRoot(IWebHostBuilder builder)
         var fromFile = File.Exists("MvcTestingAppManifest.json");
         var contentRoot = fromFile ? GetContentRootFromFile("MvcTestingAppManifest.json") : GetContentRootFromAssembly();
 
-        if (contentRoot != null)
+        if (contentRoot != null && Directory.Exists(contentRoot))
         {
             builder.UseContentRoot(contentRoot);
         }