Add a test for starting the legacy VPN.

The legacy VPN has, among many parameters, a host to connect to.
This host can be specified as a numeric address, or as a hostname.
When it's a name, resolution is required. Currently, name
resolution is performed by the native VPN daemons racoon and
mtpd. When a hostname is used, the framework does not know the
IP address of the VPN server and does not add a throw route for
the VPN server IP address. On older kernels this does not matter
because the legacy PPP kernel code binds the PPP socket to the
right network, but on newer devices that use the upstream PPP
code, this does not work. See b/133797637.

This patch instruments the legacy VPN code so that it can be
run in tests, and uses this instrumentation to simulate passing
a configuration that contains a host, and verifies that the
arguments passed to the mptd and racoon daemons receive the
expected server address, and that the expected throw route is
correctly installed.
It then adds two tests : one specifying the server as a numeric
address, and one as a hostname. As the resolution is currently
broken, the latter of these tests is added disabled, and the
followup fix to the issue enables it.

This test is basic and very targeted, but it's what we need right
now. Also there are plans to remove this entire code path in S, so
the test being ad-hoc is not much of a problem.

Test: this
Bug: 158974172
Change-Id: I420b63db03a58e8c67a98a85c9f4c2d6ed7ad078
(cherry picked from commit 45846a03051a13d667732288e457eab4336a691e)