Add Daemon restart test along with Daemon utils

This PR allows writing tests that need a daemon restart.
There is also a very useful test case that can also serve as an example on how to use it.

Ping @unclejack @tianon @vieux
Can we wrap `$DEST` and `$pid` in quotes here, just in case?  (ie, `for pid in $(find "$DEST" -name docker.pid); do` and `DOCKER_PID=$(set -x; cat "$pid")`)