Disable the dart_rfb and flutter_rfb tests

[srawlins]

Work towards dart-lang/linter#5063 and dart-lang/linter#5001

There are open PRs for blocked Dart SDK work in these two repos:

• Disable two lint rules which are being removed. Goddchen/dart-rfb#26
• Disable two lint rules which are being removed. Goddchen/flutter-rfb#24

@Goddchen has not responded to these in 72 hours. This change disables the tests. The tests can be re-enabled when those PRs have been merged, and the subsequent relevant commits in this repo bumped.

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 the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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