Explicitly set tmux window size in tests.

Recent versions of tmux master gave us an 80x23 window.  tmux has been
fixed to default to 80x24, and also to allow setting window sizes in
control mode.  These flags fortunately do not conflict with older
versions tmux.  So we use them to avoid problems going forward.

src/tests/e2e-test

@@ -244,7 +244,10 @@ for run in $server_tests; do
     # XXX this ignores $TMPDIR, because it results in an overlong pathname on OS X
     tmux_socket="/tmp/.tmux-mosh-test-$$"
     ln -fs "${tmux_socket}" "${test_dir}/tmux-socket"
-    ${tmux_stdin} tmux -f /dev/null -S "${tmux_socket}" -C new-session "${srcdir}/print-exitstatus ${client_wrapper} ${sut} ${server_wrapper} \"${PWD}/${test_dir}/${run}\" \"${PWD}/${test_script} ${run}\"" > "${test_dir}/${run}.tmux.log"
+    # tmux <= 2.5 ignore -x/-y, but the client sets the session to 80x24.
+    # tmux from 2017-05-27 and later should default to an 80x24 session,
+    # but do use -x/-y on control-master clients.
+    ${tmux_stdin} tmux -f /dev/null -S "${tmux_socket}" -C new-session -x 80 -y 24 "${srcdir}/print-exitstatus ${client_wrapper} ${sut} ${server_wrapper} \"${PWD}/${test_dir}/${run}\" \"${PWD}/${test_script} ${run}\"" > "${test_dir}/${run}.tmux.log"
     rv=$?
     rm -f "${tmux_socket}" "${test_dir}/tmux-socket"
     if [ $rv -ne 0 ]; then