selftests: openvswitch: parse trunc action

The trunc action was supported decode-able but not parse-able. Add
support for parsing the action string.

Reviewed-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Link: https://patch.msgid.link/20240704085710.353845-10-amorenoz@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>