Use new `use_mockito_codegen` lint

[CoderDake]

Add the new lint and update the way we do mocks to match it's needs.

[srawlins]

There is no use_mockito_codegen lint in the Dart analyzer or Flutter linter rules. DevTools has already transitioned to using Mockito code generation via @GenerateNiceMocks in packages/devtools_test/lib/src/mocks/generated.dart and dt generate-code. Closing this issue as obsolete.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!