ListeningPortsTest: fix retry logic

In the rewrite of ListeningPortsTest (commit 8ed56018345305650eebe4069dbce0c8086a9d8d),
we stopped using ListeningPortsAssertionError for our assertion failures.
This prevented the UDP retry logic in assertNoRemotelyAccessibleListeningUdpPorts
from being effective.

Throw a ListeningPortsAssertionError instead of calling fail to allow
the retry logic to work again.

Bug: 11236346
Change-Id: I4912343a77a44e2cb1dea7961bf3d5f48cf6541a
1 file changed