Narrows down the exception check in RouterSimulationTestCase

This patch modifies the exception check in
RouterSimulationTestCase to check if the thrown exception is
ArpTimeoutException.

This covers the unit test for MN-3007.

Change-Id: I516c6a5c8edd7af26b195a7f2859d76108625c3f
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge "Narrows down the exception check in RouterSimulationTestCase"