tests/ofproto-dpif: Use vlog to test dpif behaviour.

Previously we made heavy use of the 'ovs-appctl dpif/dump-flows' command
to verify that flows were being installed into the datapath correctly.
However this is sensitive to timing, particularly when the behaviour of
revalidator threads is modified. A common race condition involves flows
being revalidated and deleted before the test script gets a chance to
connect to ovs-vswitchd and dump the datapath flows.

This patch reworks the tests to make use of the vlog facility for dpif,
checking for flow installation and flow dump messages. Most tests are
converted to check for flow installation, which should reduce these race
conditions significantly. For tests that require packet counts, these
will continue to check for flow_dump messages. Race conditions for these
should be reduced but not eliminated.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>