tests: Make test result more predictable.

The test 'ofproto-dpif - in place modification (vlan)' fails often
due to miss handling. Hence, make it more predictable by specifying
that misses should just be dropped.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Simon Horman <simon.horman@netronome.com>

tests/ofproto-dpif.at

@@ -8766,7 +8766,8 @@ OVS_VSWITCHD_START(
   [add-port br0 p1 -- set Interface p1 type=dummy ofport_request=1])
 AT_CHECK([ovs-appctl vlog/set dpif:dbg dpif_netdev:dbg])
 
-AT_CHECK([ovs-ofctl del-flows br0])
+# Always drop misses
+AT_CHECK([ovs-ofctl -O OpenFlow11 mod-table br0 all drop])
 AT_CHECK([ovs-ofctl add-flow br0 in_port=1,actions=output:local])
 
 ovs-appctl vlog/set PATTERN:ANY:'%c|%p|%m'