test-serve: wait before killing to always hide the "killed!" message.
Without the sleeps slower systems never display never display "killed!"
while faster systems sometimes or nearly always display it.
% Without -v
access log created - .hg/hgrc respected
% With -v
listening at http://localhost/
% With --prefix foo
listening at http://localhost/foo/
% With --prefix /foo
listening at http://localhost/foo/
% With --prefix foo/
listening at http://localhost/foo/
% With --prefix /foo/
listening at http://localhost/foo/