tests: mcast-snooping: Stop time for the group protocol test.

Otherwise, it randomly fails due to age not being zero under load:

  tests/mcast-snooping.at:645: ovs-appctl mdb/show br0
  --- -
  +++ /at-groups/2592/stdout
  @@ -1,5 +1,5 @@
    port  VLAN  protocol  GROUP                Age
  -    1     0  IGMPv1    224.1.1.1           0
  +    1     0  IGMPv1    224.1.1.1           1
       1     0  IGMPv2    224.1.1.2           0
       1     0  IGMPv3    233.54.12.230           0

Fixes: b222593 ("mcast-snooping: Add group protocol to mdb/show output.")
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Simon Horman <horms@ovn.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

tests/mcast-snooping.at

@@ -622,6 +622,8 @@ AT_CHECK([
     other-config:hwaddr=aa:55:aa:55:00:01 ofport_request=1 \
 ], [0])
 
+AT_CHECK([ovs-appctl time/stop])
+
 # Send IGMPv1 report packet.
 AT_CHECK([
     ovs-appctl netdev-dummy/receive p1  \