FlowsInvalidationTestCase: fix idle exp. test

idle exp test was incorrectly using real flow timeouts and waiting
ad-infinitum, thus succeeding after one minute.

Fixed the test to make it follow the same structure as the hard time
expiration test. This makes this test class run a few seconds, instead
of over a minute.

Change-Id: I97e927722ad9b4c0f643a6207bc860adffe92db4
Signed-off-by: Guillermo Ontanon <guillermo@midokura.jp>
Merge "FlowsInvalidationTestCase: fix idle exp. test"