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