[tests] Enum shorthands == tests for constructors and static methods.

Add tests similar to what we already have for ==, but with constructors and static methods.

Fix some prior == const tests.

Bug: #57038
Change-Id: Ia6afde353fd16d7dcd767d0514484cae5ac1730b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401980
Reviewed-by: Erik Ernst <[email protected]>
Commit-Queue: Kallen Tu <[email protected]>

Author: kallentu