tests: bridge: Add regression test for gif(4) interface as bridge member

if_bridge(4) should not prohibit adding a gif(4) interface which is
assigned with IP addresses as a bridge member regardless of the setting
of sysctl variable net.link.bridge.member_ifaddrs.

Assigning IP addresses on the gif(4) interface should not be affected
no matter whether it is member of a bridge interface or not.

PR: 227450
Reported by: Siva Mahadevan <me@svmhdvn.name>
Reviewed by: ivy (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52200

(cherry picked from commit b944c8ff17971b1c8f61d5612eac146f5d589e87)