In the recent federation of the plugin's mobile implementations, many tests were duplicated from what is now the app-facing package to the platform implementations. We should audit what's in the those tests and eliminate duplicates where possible.

For instance, RunnerTests and RunnerUITests should almost certainly not still exist in the app-facing package, because they should be for testing native-code-specific behaviors (it's possible there are exceptions, which is why it needs to be an audit, but my assumption is that they should all go); the same is likely true of native Java tests. Dart integration-tests likely are more likely to be useful to duplicate (app-facing to get true end-to-end testing, implementation to get local testing so we find issue before publishing), but should get a check.

/cc @mvanbeusekom