implement `fail-if`

[jaypipes]

Allow the test author to specify a condition on which the test spec should fail (and stop retrying).

For example, let's say we want to create a Deployment and check that it gets to Complete, but that if the Deployment ever shows as failing to make progress, we should fail the test spec:

name: deployment-happy-path
description: create, get, delete a Deployment
fixtures:
  - kind
tests:
  - name: create-deployment
    kube:
      create: testdata/manifests/nginx-deployment.yaml
  - name: deployment-has-2-replicas
    timeout:
      after: 20s
    kube:
      get: deployments/nginx
    assert:
      conditions:
        Progressing:
          status: true
          reason: NewReplicaSetAvailable
    fail-if:
      conditions:
        Progressing: false
  - name: delete-deployment
    kube:
      delete: deployments/nginx