selftests: net: test IPV6_TCLASS

Test setting IPV6_TCLASS via setsockopt and cmsg
across socket types.

Output without the kernel support (this series):

  Case TCLASS ICMP cmsg - packet data returned 1, expected 0
  Case TCLASS ICMP cmsg - rejection returned 0, expected 1
  Case TCLASS ICMP diff - pass returned 1, expected 0
  Case TCLASS ICMP diff - packet data returned 1, expected 0
  Case TCLASS ICMP diff - rejection returned 0, expected 1

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>