Fixed that DiagnosticVerifier.Helper sorts result Diagnostic objects …

…only by SourceSpan, meaning that when multiple source files are involved, VerifyDiagnostic and VerifyCodeFix can report false negatives, since they both rely on Diagnostic and DiagnosticResult objects being sorted the same, in order to successfully check for differences.

src/VisualStudio.Roslyn.SDK/Roslyn.SDK/ProjectTemplates/CSharp/Diagnostic/Test/DiagnosticVerifier.Helper.cs

@@ -1,4 +1,4 @@
-using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Text;
@@ -92,7 +92,10 @@ protected static Diagnostic[] GetSortedDiagnosticsFromDocuments(DiagnosticAnalyz
         /// <returns>An IEnumerable containing the Diagnostics in order of Location</returns>
         private static Diagnostic[] SortDiagnostics(IEnumerable<Diagnostic> diagnostics)
         {
-            return diagnostics.OrderBy(d => d.Location.SourceSpan.Start).ToArray();
+            return diagnostics
+                .OrderBy(d => d.Location.SourceTree.FilePath)
+                .ThenBy(d => d.Location.SourceSpan.Start)
+                .ToArray();
         }
 
         #endregion