fix overwriting of namespaced model test annotations with non-namespaced model annotations

the test-file specific find_test_file method was matching namespaced model files when annotating non-namespaced models; using the same file resolution method for all file types fixes this.

e.g: with a User and a Foo::User model, User annotations were applied to both test/unit/user_test.rb as well as test/unit/foo/user_test.rb (order of operation was a factor here - if the non-namespaced model was annotated first this effect was not seen, as the namespaced annotations would overwrite and hide the issue)

Also, switched "mock" for "double" to kill deprecation warnings in specs