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>

src/tests/local.test

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -eu
+out=$(./inpty ../../scripts/mosh --client="../frontend/mosh-client" --server="$PWD/../frontend/mosh-server" --local --bind-server=127.0.0.1 127.0.0.1 -- printf 'he%s\n' llo)
+case "$out" in
+    *hello*) exit 0;;
+    *) exit 1;;
+esac