Test a VPN with an underlying network that does not yet exist.

This test checks that if a VPN declares an underlying network
that does not exist, the capabilities of that network are applied
to the VPN as soon as the network starts to exist.

Bug: 172870110
Test: test-only change
Change-Id: Icc0701cb4cea7d91f7738c1e426e94cd26686b74
Merged-In: Icc0701cb4cea7d91f7738c1e426e94cd26686b74