package:test has a --run-skipped flag that allows ignoring the skip attribute on tests. In VS Code I've started using this to make it easier to see whether skipped tests are passing or not without having to modify the source, though it's not currently supported for Flutter.

This PR adds the same flag to flutter test and just passes it through. The test verifies it's passed through in the same way --start-paused is.

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 Flutter Style Guide, including Features we expect every widget to implement.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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