Tests: use "kill -0" for app existence check in NS tests

Tests: use "kill -0" for app existence check in NS tests

Removing the sleep 0.5 we hit a race where the "-f /proc/$pid_app"
test returns false immediately. Using "kill -0" protects us against
such races. From kill (2):

If sig is 0, then no signal is sent, but existence and permission
checks are still performed; this can be used to check for the
existence of a process ID or process group ID that the caller is
permitted to signal.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>