Conformance tests for channel subscriber status requirements

[aliok]

Problem

Following tasks are created according to the spec.

Note: There is a separate ticket for CRD conformance: #2989

Tests:

• v1alpha1: Each subscription to a channel is added to the channel status.subscribableStatus.subscribers automatically. The ready field of the subscriber identified by its uid MUST be set to True when the subscription is ready to be processed.
• v1beta1: Each subscription to a channel is added to the channel status.subscribers automatically. The ready field of the subscriber identified by its uid MUST be set to True when the subscription is ready to be processed.

Note: As there's no way of knowing what channel spec version the channel supports, instantiate a channel and then check the annotation messaging.knative.dev/subscribable in the channel instance . There's more info for this in #2987.

Persona:
System Integrator, Contributors

Exit Criteria
Test required behavior of channel, maybe report on optional capabilities.

Time Estimate (optional):
1d

[aliok]

/area channels
/area test-and-release

[aliok]

/assign @aliok