Add a trivial test that the mosh script works without needing tmux

None of the previous tests even ran the mosh script unless tmux ≥ 1.8 is
installed.  Thus ‘make check’ was “passing” on, e.g., RHEL 6 even though
its Perl is too old (5.10.1).

Signed-off-by: Anders Kaseorg <andersk@mit.edu>