[tool] Check Java integration test configuration

[stuartmorgan-g]

trafficstars

Rather than ignoring files by filename convention (*ActivityTest.java), look for the @DartIntegrationTest annotation that the test filter actually uses, to prevent accidentally excluding files.

Also validate that the annotation is present when it should be, so that forgetting to add it doesn't cause unexplained hangs when running locally.

Pre-launch Checklist

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

[auto-submit[bot]]

auto label is removed for flutter/packages, pr: 3499, due to - The status or check suite Mac_arm64 ios_platform_tests_shard_3 master has failed. Please fix the issues identified (or deflake) before re-applying this label.

[stuartmorgan-g]

Overriding stale tree status.