tests: Tweak lxc-test-ubuntu to be faster

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
tests: Speed up lxc-test-usernic

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
tests: Fix shutdown test

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