Ensure test network being torn down

Ikev2VpnTest uses TestNetworkRunnable to start test networks
to perform VPN related tests. In some low performance devices,
such as cuttlefish, the request to disconnect test network
at the end of the previous test may not be executed completely
yet. It results in the disconnect-pending network being used
by the follow-up test and fail the test. To fix this, the utility
method should wait for onLost callback before performing next
test.

Bug: 293239828
Test: atest CtsNetTestCases:android.net.cts.Ikev2VpnTest \
      --rerun-until-failure 100
Change-Id: I6f3c4f45d3dbadc7e8783c05d6cf67f0896ffa95