tests: if_gif: Add tests to cover all IPv[46] in IPv[46] setups

[zlei]

Summary

While here, rename the basic test to 4in4 to be consistent with other
tests.

MFC after: 1 week

Test Plan

# kyua test -k /usr/tests/sys/net/Kyuafile if_gif
if_gif:4in4  ->  passed  [0.166s]
if_gif:4in6  ->  passed  [0.140s]
if_gif:6in4  ->  passed  [0.172s]
if_gif:6in6  ->  passed  [0.139s]

Results file id is usr_tests_sys_net.20250727-050415-764154
Results saved to /root/.kyua/store/results.usr_tests_sys_net.20250727-050415-764154.db

4/4 passed (0 broken, 0 failed, 0 skipped)