[Dot Shorthands] Add co19 tests for "static access shorthand" feature

[sgrekhov]

Specification https://github.com/dart-lang/language/blob/main/working/3616%20-%20enum%20value%20shorthand/proposal-simple-lrhn.md

[kallentu]

Added this to the enum shorthands project.
Thanks for working on this @sgrekhov .