Add integration test demonstrating use of PersistentVolumeClaims for inter-build caching

tests/cache-volume/0-pvc.yaml

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+# Copyright 2018 The Knative Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+kind: PersistentVolumeClaim
+apiVersion: v1
+metadata:
+  name: build-cache-claim
+spec:
+  accessModes:
+  - ReadWriteOnce
+  resources:
+    requests:
+      storage: 1Gi

tests/cache-volume/1-test.yaml

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+# Copyright 2018 The Knative Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+apiVersion: build.knative.dev/v1alpha1
+kind: Build
+metadata:
+  name: test-cache-volume-1
+  labels:
+    expect: succeeded
+spec:
+  steps:
+  # Populate the persistent volume with some content.
+  - name: populate-cache
+    image: ubuntu
+    command: ['bash']
+    args: ['-c', 'echo hello > /cache/foo']
+    volumeMounts:
+    - name: build-cache
+      mountPath: /cache
+  volumes:
+  - name: build-cache
+    persistentVolumeClaim:
+      claimName: build-cache-claim

tests/cache-volume/2-test.yaml

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+# Copyright 2018 The Knative Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+apiVersion: build.knative.dev/v1alpha1
+kind: Build
+metadata:
+  name: test-cache-volume-2
+  labels:
+    expect: succeeded
+spec:
+  steps:
+  # Check that the persistent volume contains the content produced by the
+  # previous build.
+  - name: cache-populated
+    image: ubuntu
+    command: ['bash']
+    args: ['-c', '[[ $(cat /cache/foo) == "hello" ]]']
+    volumeMounts:
+    - name: build-cache
+      mountPath: /cache
+  volumes:
+  - name: build-cache
+    persistentVolumeClaim:
+      claimName: build-cache-claim