An interop test of Authenticated Header (AH) using sha2_256

This uses IKEv1. It requires xfrm

This test requires strongswan 5.1.1 or newer for AH support

This test doesn't work with strongswan 5.5.1 .. 5.5.3 see
https://wiki.strongswan.org/issues/2347

For strongswan 5.6.0, not specifying an IKE line leads to a new
error: we require PFS but Quick I1 SA specifies no GROUP_DESCRIPTION

To work around this strongswan bug, we configure pfs=no
