Test dynamically calculated VPN MTUs

This change adds testing for VPN MTU calculations, and in particular
cases where IPv6 is not supported due to MTUs being too small.

Bug: 245612827
Test: atest FrameworksNetTests:VpnTest
Change-Id: Ib064e6670244cf3b53b5d03bd62c86b2139ff32a
1 file changed