tests/netgraph: Add a test for races between if_detach() and vnet_if_return()
A ng_eiface(4) or physical interface does not involve the cloner hence
the detaching is a bit different with epair(4). Add more tests to cover
that.
PR: 292993
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D56609
(cherry picked from commit 0988abd52352ae0977cd3e5c10316b7d94e1cac8)