test_push_notifications: Close event loops.
test_push_notifications: Close event loops.

Fixes “ResourceWarning: unclosed event loop <_UnixSelectorEventLoop
running=False closed=False debug=False>”.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,zulip/zulip,kou/zulip