[pigeon] Consolidate main Dart unit tests (flutter#4641)

Eliminates `flutter_null_safe_unit_tests`, moving the tests into the same consolidated test plugin that we are using for integration tests. This removes duplicate generation of `pigeons/` Dart output, and makes the overall test harness more consistent (e.g., we can now remove some bespoke generation code in the test scripts).

(`mock_handler_tests` are still separate; that will be left for another PR.)

This also removes analysis and Dart unit testing from the set of tests run by `run_tests.dart` (the backing for custom-tests in CI) since, as checked-in packages, they are already analyzed and tested by the normal CI steps that do Dart analysis and testing, so that was duplicating CI work. They are still part of `test.dart` for convenience of local testing during Pigeon development.

Author: stuartmorgan-g