[DWDS] Restructure tests and use `package:mockito` to generate mocks

[elliette]

Background context:

Currently the majority of tests in DWDS are integration tests using chromedriver. We have a few unit tests as well, some of which implement their own fakes (eg, in metadata_test.dart), others of which use the fakes from fakes.dart.

Suggested improvements:

• separate directories for integration and unit tests
• switch from the fakes in fakes.dart to mocks generated by package:mockito
• move fakes from other test files into fakes.dart so that they can easily be found
• increase number of unit tests
• add documentation / general guidelines on writing and running tests

[bkonyi]

After talking with @elliette, it doesn't seem like we've made any progress on this issue. We should consider whether or not we actually want to use package:mockito since it involves a lot of code generation steps since Dart 3.0.