Improve CustomAssert.AreXmlEquivalent fail message

... so we have a better chance at finding out why one of the tests rely-
ing on it is flaky on AppVeyor.

Author: stakx

src/Castle.Core.Tests/Components.DictionaryAdapter.Tests/CustomAssert.cs

@@ -27,7 +27,14 @@ public static void AreXmlEquivalent(XmlElement expected, XmlElement actual)
 			if (expected == null) throw new ArgumentNullException("expected");
 			if (actual == null) throw new ArgumentNullException("actual");
 
-			Assert.True(Matches(expected, actual));
+			if (Matches(expected, actual))
+			{
+				Assert.Pass();
+			}
+			else
+			{
+				Assert.Fail("XmlElements are not equivalent.\nExpected: `{0}`\nActual:   `{1}`", expected.OuterXml, actual.OuterXml);
+			}
 		}
 
 		public static void AreXmlEquivalent(string expected, XmlElement actual)