netlink: add support for decoding genl ops/groups in pytest

MFC after: 2 weeks

tests: fix utils import in netlink tests

MFC after: 2 weeks

tests: add support for parsing generic netlink families.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39370

tests: split netlink.py into multiple files to impove maintainability.

This diff does not contain any functional changes.
Its sole purpose is splitting netlink.py into smaller chunks.
The new code s

tests: split netlink.py into multiple files to impove maintainability.

This diff does not contain any functional changes.
Its sole purpose is splitting netlink.py into smaller chunks.
The new code simplifies the upcoming generic netlink support
introduction.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39365

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