test_push_notifications: Fix incorrect "read" flag test.
test_push_notifications: Fix incorrect "read" flag test.

It was unclear what the original test was testing, and more
importantly, the test passed even if one removed the `read` flag
check in the `handle_push_notifications` function, so we fix it
to be more comprehensive.

rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip