[flutter_plugin_tools] Replace xctest and java-test with native-test

[stuartmorgan-g]

Creates a new native-test command that will be used to run native unit and UI/integration tests for all platforms over time. This replaces both xctest and java-test.

For CI we can continue to run each platform separately for clarity, but the combined command makes it easier to use (and remember how to use) for local development, as well as avoiding the need to introduce several new commands for desktop testing as support for that is added to the tool.

Fixes flutter/flutter#84392
Fixes flutter/flutter#86489

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

It looks like the overall change broke the rename detection threshold, so it'll be easier to review the individual commits rather than the whole PR's diff.

Also, if you'd rather I split this into "convert xctest to native-test" and "fold java-test into native-test" as separate PRs I can easily do that. I was going to do it initially, but I thought seeing what it looked like with more than one platform's test might make the overall structure clearer.

[jmagman]

LGTM w/ spelling nit

[jmagman]

Suggested change

	Future<_PlatformResult> _testMacOs(Directory plugin, _TestMode mode) {
	Future<_PlatformResult> _testMacOS(Directory plugin, _TestMode mode) {

[stuartmorgan-g]

I never know what to do with iOS and macOS in Dart code :P I hadn't realized there was an exception for two-letter acronyms in Dart style though, thanks for flagging this!

[jmagman]

You don't need to address it in this PR since I know this was copied, but I just remembered about the image_picker RunnerUITestiOS14 target. It probably doesn't make sense for that to be a separate target, I wasn't involved in the recent creation of that.

[stuartmorgan-g]

Yes, I found that when checking the repo to make sure this would work, and discussed it with folks this morning; it should get folded back in in the next day or two.