test/integration/apiserver/apply covers the behavior of server-side-apply (SSA)
for official APIs. But there seem to be no integration tests which cover the
semantic of SSA like adding/removing/updating entries in a list map. This adds
such a test.
It needs an API which is under control of the test and uses
k8s.io/apimachinery/pkg/apis/testapigroup for that purpose, with some issues
fixed (OpenAPI code generation complained) and a new list map added.
Registering that API group in the apiserver needs a REST storage and
strategy. The API group only gets added in the test. However, the production
code has to know about it. In particular,
pkg/generated/openapi/zz_generated.openapi.go has to describe it.
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