Remove varargs and test context object from factory.

Due to lifetime differences, it is impractical and not recommended to set the test context for the factory during initialization, so all creation methods (MakeUser, MakeService, etc.) now take an additional *gc.C parameter.

Also, variadic parameters are exchanged for a pointer parameter for object properties - MakeUser() becomes MakeUser(c, nil). This makes the code more readable and idiomatic.