tests: fix PTAP system test to check only OF stats

tests: fix PTAP system test to check only OF stats

It turned out, checking datapath flow statistics during system-userspace
test is not reliable. Unwanted packets can be injected depending on
system configuration. As a workaround, this commit removes checking
statistics of datapath flows and does check OpenFlow statistics of the
integrator bridges. Datapath flows can be checked in normal PTAP unit
tests by running 'make check'.

Reported-by: Darrell Ball <dball@vmware.com>
Suggested-by: Jan Scheurich <jan.scheurich@ericsson.com>
Tested-by: Darrell Ball <dball@vmware.com>
Signed-off-by: Zoltan Balogh <zoltan.balogh@ericsson.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>