bots: Pass realm to get_system_bot calls in test_cache.py.
bots: Pass realm to get_system_bot calls in test_cache.py.

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