Allow test caller to construct a TestNetworkRunnable to run a test with a test network. This is a preparation for adding tests to verify kernel implementation of IPsec algorithms. Bug: 171083832 Test: atest Ikev2VpnTest Original-Change: https://android-review.googlesource.com/1503692 Merged-In: I0e08a6ea299e6ad94d69806a2d76fa155653b62a Change-Id: I0e08a6ea299e6ad94d69806a2d76fa155653b62a