test_upload: Fix some URLs to uploaded files.
test_upload: Fix some URLs to uploaded files.

Using http://localhost:9991 is incorrect - e.g. messages sent with file
urls constructed trigger do_claim_attachments to be called with empty
list in potential_path_ids.

realm.host should be used in all these places, like in the other tests
in the file.

zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip