[swift2objc] Unify test/unit/filter_test.dart and test/integration/integration_test.dart

[liamappelbe]

filter_test.dart is actually an integration test, not a unit test. And it's essentially doing the same thing as integration_test.dart. The only difference, the reason it's separate, is that it applies config filters to the generated bindings.

It would be cleaner to make the filter tests just ordinary integration tests, but have a way in integration_test.dart of customizing the config per-test. Maybe something as simple as a hard coded map declared in integration_test.dart from test name to a list of different configs to apply.