selftests: ovpn: flatten slurped notification JSON before filtering

selftests: ovpn: flatten slurped notification JSON before filtering

Notification comparison uses jq -s, which slurps all inputs into an
array. Some inputs can be arrays themselves, and applying the .msg.peer
filter directly on those entries triggers jq type errors.

Expand any array-valued JSON items returned by jq -s before selecting
.msg.peer, so the filter handles both normal notification objects and []
entries without type errors.

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>