tests: Wait 5s for init to respond in lxc-test-autostart

lxc-test-autostart occasionaly fails at the restart test in the CI
environment. Looking at the current test case, the most obvious race
there is if lxc-wait exists succesfuly immediately after LXC marked the
container RUNNING (init spawned) but before init had a chance to setup
the signal handlers.

To avoid this potential race period, let's add a 5s delay between the
tests to give a chance for init to finish starting up.

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