tests: gif(4): Add regression tests for setup with EtherIP

Tunnel IPv[46] traffic over IPv[46] should still function when the
gif(4) interface is member of a if_bridge(4) interface, aka the
EtherIP setup.

PR: 227450
Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51682