notif ios: More thorough testing for APNs payload parser

[rajveermalviya]

Add more tests for NotificationOpenPayload.parseIosApnsPayload, similar to the tests for buildAndroidNotificationUrl:

zulip-flutter/test/notifications/display_test.dart

Line 1358 in 618a75c

test('parse: fails when missing any expected query parameters', () {

See discussion: #1379 (comment)