EndpointSlice is always enabled now, so make it non-optional in the
EndpointsAdapter, make all of the test cases pass an EndpointSlice
client, and remove the "EndpointSlices disabled"-specific tests.
By changing makeEndpointsArray() to return both an Endpoints and an
EndpointsSlice, the "initialObjects" and "expectCreate"/"expectUpdate"
fields of (almost) all of the existing unit tests are automatically
switched to be EndpointSlice-aware; instead of having an initial state
with just Endpoints (or nothing), and testing that just the Endpoints
is created/updated correctly, they now have an initial state with both
Endpoints and EndpointSlice (or nothing), and test that both objects
are created/updated correctly.
The handful of existing test cases that used inconsistent Endpoints
and EndpointSlice state have been modified to create the objects
separately.
Consistently verify creates/updates based on the fake client's action
tracking, not based on the return values of the reconciler functions.
(This will also let us check that both Endpoints and EndpointSlices
were created/updated correctly.)
