tests: have dumbhttp.py use cmdutil.service() to wait for child to listen()
Because the original dumbhttp.py exited without waiting for listen(), several
tests could fail with "abort: error: Connection refused" if subsequent hg
command is fast enough.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state