Minor refactoring so that the CI can display the messages

Author: nulltoken

LibGit2Sharp.Tests/MetaFixture.cs

@@ -254,12 +254,18 @@ public void GetEnumeratorMethodsInLibGit2SharpMustBeVirtualForTestability()
                     (!m.IsVirtual || m.IsFinal))
                 .ToList();
 
-            foreach (var method in nonVirtualGetEnumeratorMethods)
+            if (nonVirtualGetEnumeratorMethods.Any())
             {
-                Debug.WriteLine(String.Format("GetEnumerator in type '{0}' isn't virtual.", method.DeclaringType));
-            }
+                var sb = new StringBuilder();
 
-            Assert.Empty(nonVirtualGetEnumeratorMethods);
+                foreach (var method in nonVirtualGetEnumeratorMethods)
+                {
+                    sb.AppendFormat("GetEnumerator in type '{0}' isn't virtual.{1}",
+                        method.DeclaringType, Environment.NewLine);
+                }
+
+                Assert.True(false, Environment.NewLine + sb.ToString());
+            }
         }
     }
 }