ovn.at: Fix "Symmetric IPv6 ECMP reply flows" test.

When running tests against OVS master branch, the "Symmetric IPv6 ECMP
reply flows" test was failing because it wasn't taking into account the
possibility that null-SNAT might be supported by the userspace datapath
(which it is, since ba16a36f358a ("dpif-netdev: Add all-zero SNAT to the
advertised features of ct.")).

Fixes: 58683a4 ("ovn-controller: Handle DNAT/no-NAT conntrack tuple collisions.")
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Acked-by: Mark D. Gray <mark.d.gray@redhat.com>
Signed-off-by: Numan Siddique <numans@ovn.org>

tests/ovn.at

@@ -23468,7 +23468,7 @@ AT_CHECK([
     for hv in 1 2; do
         grep table=15 hv${hv}flows | \
         grep "priority=100" | \
-        grep -c "ct(commit,zone=NXM_NX_REG11\\[[0..15\\]],exec(move:NXM_OF_ETH_SRC\\[[\\]]->NXM_NX_CT_LABEL\\[[32..79\\]],load:0x[[0-9]]->NXM_NX_CT_LABEL\\[[80..95\\]]))"
+        grep -c "ct(commit,zone=NXM_NX_REG11\\[[0..15\\]],.*exec(move:NXM_OF_ETH_SRC\\[[\\]]->NXM_NX_CT_LABEL\\[[32..79\\]],load:0x[[0-9]]->NXM_NX_CT_LABEL\\[[80..95\\]]))"
 
         grep table=22 hv${hv}flows | \
         grep "priority=200" | \