Split the VTI tests and the tunnel tests.

This will allow the VTI test to grow into a more complete test
that creates multiple VTIs in its setup and teardown methods, and
tests routing across and between them. It also makes things a bit
easier to read because the test is already quite large.

In order to do this:

1. Move _CreateXfrmTunnel to xfrm_base.py, since it's generic
   code that is not really specific to the tests. Arguably this
   should go into xfrm.py, but at the moment it can't because it
   doesn't take any algorithm parameters.
2. Make _GetLocalInnerAddress and friends module functions
   instead of class functions, since they don't need access to
   any test class fields or methods.

Bug: 70371070
Test: xfrm_tunnel_test.py passes on android-4.9
Change-Id: If1b6ae0b659a17e387f063421382b027580ed501