test: Log debugging output from test script

The test scripts run with sh -e, which means they will stop if any commands
return an error.  That's generally desirable, because we won't continue
after things are hopeless due to an earlier step failing.

Unfortunately, the tmux setup we run the script in means it's not obvious
where any error messages related to such a failure will go.  Depending on
exactly where the error occurs they might go to the original terminal
hidden behind tmux, or they might go to a tmux panel that's not visible in
the normal layouts.

To make it easier to find such error message, redirect direct output and
errors from the test script itself to a 'script.log' file in the logs
directory.  When in DEBUG=1 mode, additionaly 'set -x' so we log all the
commands we execute to that file.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

test/run

@@ -161,6 +161,8 @@ demo() {
 [ "$(basename "${0}")" = "run_demo" ] && DEMO=1
 
 if [ "${1}" = "from_term" ]; then
+	exec > ${LOGDIR}/script.log 2>&1
+	[ ${DEBUG} -eq 1 ] && set -x
 	cd ..
 	if [ ${DEMO} -eq 1 ]; then
 		demo