selftests: net: Test headroom handling of ip6_gre devices

Commit 5691484df961 ("net: ip6_gre: Fix headroom request in
ip6erspan_tunnel_xmit()") and commit 01b8d064d58b ("net: ip6_gre:
Request headroom in __gre6_xmit()") fix problems in reserving headroom
in the packets tunneled through ip6gre/tap and ip6erspan netdevices.

These two patches included snippets that reproduced the issues. This
patch elevates the snippets to a full-fledged test case.

Suggested-by: David Miller <davem@davemloft.net>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>