Mesh conformance tests should be careful with parentRef groups

[kflynn]

In many cases, the mesh conformance tests are applying HTTPRoutes including, for example:

parentRefs:
  - kind: Service
    name: echo-v2

The astute reader will note the absence of a group. Per the parentRefs specification:

When [group is] unspecified, "gateway.networking.k8s.io" is inferred.

This is problematic, because gateway.networking.k8s.io doesn't contain a Service. Meshes which correctly interpret parentRefs will fail these tests.

How to reproduce it (as minimally and precisely as possible):

Run the conformance tests with the HTTPResponseHeaderModification feature enabled.