tests: Test learned flow idle timeouts.

tests: Test learned flow idle timeouts.

The previous tests would check that a single learned flow had its stats
correctly attributed to the right interfaces and flows. These new tests
take it a step further by causing two different learned flows to be
created, and checking the stats are correct. This is done for rules that
are learned with idle and hard timeouts.

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