This finishes the conversion of run_tests.sh to run_tests.dart:

• Removes test_pigeon_android and its supporting logic, which has actually been dead code for quite a while. (Per offline discussion, we don't think the custom lint/error checks it used to do have enough value to rebuild in the new structure at this time. If we were to do that later, it would almost certainly take a very different form, such as Gradle plugins in test_plugin.)
• Folds the input files from test_pigeon_dart/run_dart_compilation_tests, which analyzed Dart output, into the existing Dart unit tests test, which already had the logic to analyze Dart output. This reduces the number of types of tests by one.
• Migrates test_command_line into the Dart script.
• Migrates run_ios_legacy_unittests to Dart for now, since we still haven't resolved the CI issues.
• Migrates run_android_unittests into the Dart script.
• Migrates all of the logic for what to run by default into a new Platform.isMacOS section in the Dart script, instead of the Dart script running nothing on macOS by default, since the old behavior was only to avoid the CI double-running all of the tests (once from running run_tests.sh without arguments, and once from running run_tests.dart without arguments).

This allows us to completely remove run_tests.sh, and therefore also remove the special-casing of pigeon in the Linux CI, since the Dart script can internally no-op on Linux. This PR doesn't attempt to address flutter/flutter#115393, but it does make it very easy to do so. That will be done in a follow-up PR.

Fixes flutter/flutter#85068

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. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• 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.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.