if_gre: Add a regression test

[pouria]

Summary

Add a regression test for gre(4)
to make sure all of the gre capabilities and options are
working as intended.

This test depends on PR275474 to be resolved.
otherwise it's going to trigger kernel panic.

Test Plan

To test, without triggering kernel panic, add the following lines below on top of every cleanup function:

sleep 1
ifconfig -j gretest1 gre1 destroy
sleep 1
ifconfig -j gretest2 gre1 destroy
sleep 1

and then run the tests:

# kyua test -k /usr/tests/sys/net/Kyuafile if_gre
if_gre:gre4_basic  ->  passed  [0.145s]
if_gre:gre6_basic  ->  passed  [0.148s]
if_gre:gre6_csum  ->  passed  [0.149s]
if_gre:gre6_key  ->  passed  [0.153s]
if_gre:gre6_seq  ->  passed  [0.160s]
if_gre:gre6_udpencap  ->  passed  [0.149s]
if_gre:gre_blind_options  -> passed [0.157s]