[_fe_analyzer_shared] Add test for named mixin application in macro metadata

Change-Id: Ia968e49f2663970496ba289c5098802e1794799a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391760
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>

Author: johnniwinther