test_realm: Fix test_do_deactivate_realm_clears_user_realm_cache.
test_realm: Fix test_do_deactivate_realm_clears_user_realm_cache.

self.example_user("hamlet") uses get_user_by_delivery_email, so it
doesn't actually cache anything. This should use a cached function, like
the test below: test_do_change_realm_subdomain_clears_user_realm_cache.

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