Bug 996: Fixed UT bug due to incorrect VTN event synchronization.

If the test code verifies the number of VTN events delivered to the
listener instance, it needs to ensure that no VTN event is pending
before adding the listener.

Additional changes:

  * VTNThreadDataTest: Fixed too short timeout.
  * Add some test cases for IVTNModeListener.

Change-Id: Ic6efdab85e9cdbda9f73d27bf1319263edba67ab
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>