Request Test Networks for VCNs when in Test Mode.

This CL updates UnderlyingNetworkTracker to request Test
Networks for route selection  when the VCN it is tracking
for is in test-mode.

Bug: 182291467
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: Icb59b15df61ec43fe76e250977a985caa6a1dffc