[Static Extensions] Add co19 tests for the Static Extensions feature

This is a meta-issue for the tests for the Static Extensions language feature.

_The language feature is currently being specified. This is a provisional issue and may not be actionable until the specification is written. The current draft of the specification can be found here: https://github.com/dart-lang/language/blob/main/working/0723-static-extensions/feature-specification.md._