tests: Try to make shutdowntest a bit more robust

This should help it run better on slow test environment like the LXC CI
armhf builder.
 - Wait longer for the container to start
 - Wait longer for the container to shutdown
 - On failure to shutdown, kill the container
 - Always destroy the container if it's around

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>