Adding IOS Safari unit tests to flutter web

rerun after setupxcode is added
recipe running on linux: https://chromium-swarm.appspot.com/task?id=4cfcd5434cca9d10
recipe running on mac: https://chromium-swarm.appspot.com/task?id=4cfcc708de93b210

Change-Id: I7dafa9490279ac4c701b212335870c0e2bfcd9f6
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/3900
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Nurhan Turgut <nurhan@google.com>