This creates a new test.dart script for developers to run tests locally, separate from the run_tests.dart entrypoint whose no-argument behavior is constrained by our CI requirements since it's what the repo tooling's custom-tests runs.

Most of the logic is extracted to a new shared file, which is used by run_tests.dart and test.dart.

• run_tests.dart is now solely intended for CI. It distributes the tests among the host platforms so that everything is run, but not all on the same host, and not more than once. It follows the same logic we use for most of our CI in that it runs everything it can on Linux, and only the things that need to be run on Windows and macOS are run there. It also adds validation that we are running every test suite on at least one host unless explicitly opted out, so that we can't accidentally forget to run things in CI (and so that we can easily see in one place what we aren't running in CI).
• test.dart is for developers to run locally. It still has the arguments for selecting specific tests, but the default behavior is to run every test that can run on that host, so local runs get the maximum possible coverage. This includes integration tests that we aren't currently set up to be able to run in CI. This means developers don't need to remember what is and isn't run locally by default.

Linux custom-tests is moved to the heavy workload machine configuration in the Cirrus configuration since the Android build is OOMing on the light workload configuration.

Fixes flutter/flutter#115393

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.