Add VpnTestBase for VPN test common code

This is a no-op change to move some common codes that share
between different VPN unit tests to the new base class.
It's helpful to reduce the code duplication for the follow up
work to add the VpnManagerServiceTest and other test codes
refactor.

Bug: 231373589
Test: atest FrameworksNetTests
Change-Id: Id5111e7ca33ea8f9eb4bcc6c13d0e681f0664d24
2 files changed