Add test-geard-image command

Add initial implementation to test an image.

This command expects that the git repository for the image contains
.sti/bin/test executable and a test-app directory that contains a sample
application that STI can build.

.sti/bin/test should execute the main command (e.g. /opt/sti/bin/run) in
the background and then proceed to perform smoke tests against the
container.