[CHANGE] test.sh now accepts commit to test from stdin; only supports rubble.

Now you can pass -s to test.sh, which will make it read key=value pairs from
stdin. When stdin is closed, the script will continue as normal. The only
key/value pair that is relevant right now is:

    DS_COMMIT=<git commit hash>

Which now causes this script to do a git checkout at that commit hash, rather
than a git pull.

Also, passing 'rubble' as an argument is now redundant. The rubble test
framework is always triggered.