Test harness should support asserting that a specific number of a resource exist when listing

[oliviabarrick]

What would you like to be added:

In the test harness, you can match objects without specifying a name, for example:

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: nginx
status:
  state: READY

It should be possible to specify how many objects match.

Why is this needed:

This can be used to verify that a certain number of pods are ready or a certain number of plan executions exist.

[oliviabarrick]

Depending on how we design this, this may be the same as kudobuilder/kudo#494