Made DatabaseEnabledTestsBase.Cleanup virtual.

This enables child classes to provide additional cleanup.

src/Orchard.Tests.Modules/DatabaseEnabledTestsBase.cs

@@ -58,7 +58,7 @@ namespace Orchard.Tests.Modules {
         }
 
         [TearDown]
-        public void Cleanup() {
+        public virtual void Cleanup() {
             if(_container != null)
                 _container.Dispose();