test_message_send: Modify test_wildcard_mention_restrictions.
test_message_send: Modify test_wildcard_mention_restrictions.

This commit modifies the test_wildcard_mention_restrictions test
for checking that moderators are not allowed to send messages
with wildcard mention if wildcard_mention_policy is set to
WILDCARD_MENTION_POLICY_ADMINS. Previously, we were checking
for members, but it is better to check for moderators.

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