selftests: ovpn: fail notification check on mismatch

selftests: ovpn: fail notification check on mismatch

compare_ntfs doesn't fail when expected and received notification
streams diverge.

Fix this bug by tracking the diff exit status explicitly and return it
to the caller so notification mismatches propagate as test failures.

Fixes: 77de28cd7cf1 ("selftests: ovpn: add notification parsing and matching")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>