Fixture builders for Subscription reconcile tests

[grantr]

The existing fixture builders in the Subscription reconciler tests aren't terrible, but they don't compose well. Every case needs its own fixture method like getNewSubscriptionWithReferencesResolvedAndPhysicalReplyAndNoSubscriber.

@dprotaso experimented with builders in knative/serving#1762 and knative/serving#1924, but apparently moved on to other things before getting to a satisfactory state.

This is my attempt at a builder interface satisfying the following:

• minimal code for both framework and test authors to write and understand
• Flexible for test authors to define their own builder function signatures
• No runtime typing (more than what's already used by the controller-runtime client)
• As clear (or clearer) to read as the existing fixtures

I chose the Subscription reconcile tests for this PoC because they use more complex combinations. The diffstats are probably a best case scenario for this kind of refactor.

Here's the diff for the above fixture invocation:

   WantPresent: []runtime.Object{
-    getNewSubscriptionWithReferencesResolvedAndPhysicalSubscriberAndNoReply(),
+    Subscription().ReferencesResolved().PhysicalSubscriber(targetDNS).EmptyNonNilReply(),
   },

And an example of one of the builder methods:

func (s *SubscriptionBuilder) EmptyNonNilSubscriber() *SubscriptionBuilder {
	s.Spec.Subscriber = &eventingv1alpha1.SubscriberSpec{}
	return s
}

@mattmoor

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[grantr]

/cc @vaikas-google @Harwayne

[Harwayne]

/lgtm

Looks much nicer.