email_mirror: Clean up and expand test_get_missed_message_token.
email_mirror: Clean up and expand test_get_missed_message_token.

This is the only function in TestEmailMirrorLibrary, so we rename this
class to more appropriate TestGetMissedMessageToken, clean it up a bit
and add some extra checks to finally get email_mirror.py to 100% test
coverage.

timabbott/zulip,rht/zulip,synicalsyntax/zulip,rishig/zulip,punchagan/zulip,rishig/zulip,rht/zulip,showell/zulip,timabbott/zulip,tommyip/zulip,punchagan/zulip,showell/zulip,andersk/zulip,showell/zulip,zulip/zulip,rishig/zulip,rht/zulip,tommyip/zulip,showell/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,tommyip/zulip,brainwane/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,tommyip/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,timabbott/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,rishig/zulip,timabbott/zulip,shubhamdhama/zulip,synicalsyntax/zulip,kou/zulip,kou/zulip,synicalsyntax/zulip,showell/zulip,shubhamdhama/zulip,synicalsyntax/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,brainwane/zulip,kou/zulip,shubhamdhama/zulip,eeshangarg/zulip,timabbott/zulip,tommyip/zulip,eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,rht/zulip,zulip/zulip,brainwane/zulip,tommyip/zulip,shubhamdhama/zulip,zulip/zulip,andersk/zulip,synicalsyntax/zulip,rishig/zulip,brainwane/zulip,andersk/zulip,rishig/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip