ovn test: remove check for non-existing bridge in hv3

ovn test: remove check for non-existing bridge in hv3

In OVN vtep test, the network topology is like this:

  hv1---\
         >-- [net1] <-- vtep --> [net2] <-- hv3
  hv2---/

The logical switch lsw0 created in this test has no logical
port corresponding to hv3, so that hypervisor does not have
any bridges created by OVN. With this test change, we are
replacing the 'show br-int' with a check to ensure that
'br-int' is not present.

Fixes: 8dab102238f0 ("ovn: Add more details to test output.")
Signed-off-by: Flavio Fernandes <flavio@flaviof.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>