Included the TestVirtualFunctionRetVal for Linux (#846)

Fixes #755

It turns out this wasn't a bug in our code because the test passes now with no changes whatsoever. My only guess is that there used to be a bug in Mono.

tests/VTables/VTables.Tests.cs

@@ -55,7 +55,7 @@ public class VTablesTests : GeneratorTestFixture
         Assert.AreEqual(actual, ret);
     }
 
-    [Test, Platform(Exclude = "Linux", Reason = "Virtual tables are incorrectly patched with 64-bit Itanium, see https://github.com/mono/CppSharp/issues/755.")]
+    [Test]
     public void TestVirtualFuntionRetVal()
     {
         // Virtual Functions Object Slicing case