Improving EndpointSlice tests

This adds 2 new tests covering EndpointSlices, including new coverage of
the self referential Endpoints and EndpointSlices that need to be
created by the API Server and the lifecycle of EndpointSlices from
creation to deletion. This also removes the [feature] indicator from the
name to ensure that this test will run more often now that it is enabled
by default.

Author: robscott