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]

[markj]

tests/sys/net/if_gre.sh

50

You can now set required.kmods in the ATF header instead.

[pouria]

Address @markj comment.

[markj]

tests/sys/net/if_gre.sh

35

It should be require.kmods, my comment had a typo.

37

IMO it is nicer to group test subroutines together without whitespace in between:

atf_test_case foo
foo_head()
{
}
foo_body()
{
}
foo_cleanup()
{
}

This requires less vertical space, and makes it easier to visually group the test subroutines together.

59

-s exit:0 is the default, you don't need to write it everywhere.

418

What does "blindly" mean here?

[pouria]

Address @markj comments.
Thank you for all of your great reviews.

[pouria]

tests/sys/net/if_gre.sh

35

It should be require.kmods, my comment had a typo.

Oops, didn't think/test when I updated the code. :(
sorry

418

I don't verify its actual effect on dataplane here.
We simply want to make sure this options works and driver updates itself, regardless of using netlink or ioctl.
For example, I want to make sure that driver knows I've set the udpport to x to make sure my netlink implementation works or at least it won't break without failing a test.

I've done the same thing in if_geneve due its large number of options and it helps A LOT!