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

### 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`](https://github.com/dart-lang/webdev/blob/ab82887641d794d00d7d510e4f3d005c491adce3/dwds/test/metadata_test.dart#L40-L56)), others of which use the fakes from [`fakes.dart`](https://github.com/dart-lang/webdev/blob/ab82887641d794d00d7d510e4f3d005c491adce3/dwds/test/fixtures/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

Start by reviewing dwds/test/metadata_test.dart and dwds/test/fixtures/fakes.dart, then inspect how the existing chromedriver integration tests and unit tests are organized. Done means separating integration and unit tests, consolidating fakes, switching to package:mockito-generated mocks, adding unit tests, and documenting how to write and run them.

Written by the indexing model from the issue text.