test(direct_imap): always pass mark_seen=False to fetch()

mark_seen=True is the default and translates to fetching BODY
of the message. mark_seen=False translates to fetching BODY.PEEK
that is described in IETF RFC 3501 as
"An alternate form of BODY[<section>] that does not implicitly set the \Seen flag."

It is unexpected unless you know this detail of IMAP protocol already,
but this is not going to be fixed in imap_tools: <https://github.com/ikvk/imap_tools/issues/179>

deltachat-rpc-client/tests/conftest.py

@@ -171,7 +171,7 @@ class DirectImap:
         self.conn.append(bytes(msg, encoding="ascii"), folder)
 
     def get_uid_by_message_id(self, message_id) -> str:
-        msgs = [msg.uid for msg in self.conn.fetch(AND(header=Header("MESSAGE-ID", message_id)))]
+        msgs = [msg.uid for msg in self.conn.fetch(AND(header=Header("MESSAGE-ID", message_id)), mark_seen=False)]
         if len(msgs) == 0:
             raise Exception("Did not find message " + message_id + ", maybe you forgot to select the correct folder?")
         return msgs[0]