Register new model class location for test double

In addition to registering the Mongo serialization options for the real
MongoConnection, we also need to do the same thing for the test double
of the MongoConnection class, so that it can test real data correctly.

src/LfMerge.Core.Tests/TestDoubles.cs

@@ -108,7 +108,7 @@ public static void Initialize()
 			ConventionRegistry.Register(
 				"My Custom Conventions",
 				pack,
-				t => t.FullName.StartsWith("LfMerge."));
+				t => t.FullName.StartsWith("LfMerge.") || t.FullName.StartsWith("LfMergeBridge.LfMergeModel"));
 
 			// Register class mappings before opening first connection
 			new MongoRegistrarForLfConfig().RegisterClassMappings();