selftests/bpf: add a test to test_align

New test adds 14 to the unknown value before adding to the packet pointer,
 meaning there's no 'fixed offset' field and instead we add into the
 var_off, yielding a '4n+2' value.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>