selftests: ovpn: add notification parsing and matching

selftests: ovpn: add notification parsing and matching

To verify that netlink notifications are correctly emitted and contain
the expected fields, this commit uses the tools/net/ynl/pyynl/cli.py
script to create multicast listeners. These listeners record the
captured notifications to a JSON file, which is later compared to the
expected output.

Cc: linux-kselftest@vger.kernel.org
Cc: shuah@kernel.org
Cc: horms@kernel.org
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>