tests: Skip "IPv6 switching - megaflow check" if scapy is not installed.

Otherwise the test blocks indefinitely.

Fixes: 2e7f318 ("Reply only for the multicast ND solicitations.")
Signed-off-by: Numan Siddique <numans@ovn.org>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit 7fd6c21)

tests/ovn.at

@@ -37901,6 +37901,7 @@ dnl don't match on IPv6 source and destination addresses for
 dnl simple switching.
 OVN_FOR_EACH_NORTHD([
 AT_SETUP([IPv6 switching - megaflow check for IPv6 src/dst matches])
+AT_SKIP_IF([test $HAVE_SCAPY = no])
 ovn_start
 
 check ovn-nbctl ls-add sw0