Create test-mode for VcnConfig.

This CL creates a test-mode for VcnConfig so that they will only
match with Test Networks. This is necessary for CTS testing so
that VCNs can run on test networks and IKE negotiations can be
injected over the Test Networks.

Bug: 182291467
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: I5cc340e5aaa34c5de8efafa52de49185a18d4bd3