kubeadm-upgrade: add unit tests for the diff command

Add the file diff_test.go, which has a single test:
  TestRunDiff

The test covers most error cases for the runDiff() function,
and also performs a valid diff.

A couple of test files are added in:
cmd/kubeadm/app/cmd/upgrade/testdata/

cmd/kubeadm/app/cmd/upgrade/BUILD

@@ -44,8 +44,10 @@ go_test(
     srcs = [
         "apply_test.go",
         "common_test.go",
+        "diff_test.go",
         "plan_test.go",
     ],
+    data = glob(["testdata/**"]),
     embed = [":go_default_library"],
     deps = [
         "//cmd/kubeadm/app/apis/kubeadm:go_default_library",

cmd/kubeadm/app/cmd/upgrade/diff_test.go

@@ -0,0 +1,93 @@
+/*
+Copyright 2018 The Kubernetes 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.
+*/
+
+package upgrade
+
+import (
+	"io/ioutil"
+	"testing"
+)
+
+const (
+	testUpgradeDiffConfig   = `testdata/diff_master_config.yaml`
+	testUpgradeDiffManifest = `testdata/diff_dummy_manifest.yaml`
+)
+
+func TestRunDiff(t *testing.T) {
+	parentFlags := &cmdUpgradeFlags{
+		cfgPath: "",
+		out:     ioutil.Discard,
+	}
+	flags := &diffFlags{
+		parent: parentFlags,
+	}
+
+	testCases := []struct {
+		name            string
+		args            []string
+		setManifestPath bool
+		manifestPath    string
+		cfgPath         string
+		expectedError   bool
+	}{
+		{
+			name:            "valid: run diff on valid manifest path",
+			cfgPath:         testUpgradeDiffConfig,
+			setManifestPath: true,
+			manifestPath:    testUpgradeDiffManifest,
+			expectedError:   false,
+		},
+		{
+			name:          "invalid: missing config file",
+			cfgPath:       "missing-path-to-a-config",
+			expectedError: true,
+		},
+		{
+			name:            "invalid: valid config but empty manifest path",
+			cfgPath:         testUpgradeDiffConfig,
+			setManifestPath: true,
+			manifestPath:    "",
+			expectedError:   true,
+		},
+		{
+			name:            "invalid: valid config but bad manifest path",
+			cfgPath:         testUpgradeDiffConfig,
+			setManifestPath: true,
+			manifestPath:    "bad-path",
+			expectedError:   true,
+		},
+		{
+			name:          "invalid: badly formatted version as argument",
+			cfgPath:       testUpgradeDiffConfig,
+			args:          []string{"bad-version"},
+			expectedError: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			parentFlags.cfgPath = tc.cfgPath
+			if tc.setManifestPath {
+				flags.apiServerManifestPath = tc.manifestPath
+				flags.controllerManagerManifestPath = tc.manifestPath
+				flags.schedulerManifestPath = tc.manifestPath
+			}
+			if err := runDiff(flags, tc.args); (err != nil) != tc.expectedError {
+				t.Fatalf("expected error: %v, saw: %v, error: %v", tc.expectedError, (err != nil), err)
+			}
+		})
+	}
+}

cmd/kubeadm/app/cmd/upgrade/testdata/diff_dummy_manifest.yaml

@@ -0,0 +1 @@
+some-empty-file-to-diff

cmd/kubeadm/app/cmd/upgrade/testdata/diff_master_config.yaml

@@ -0,0 +1,3 @@
+apiVersion: kubeadm.k8s.io/v1alpha1
+kind: MasterConfiguration
+kubernetesVersion: 1.11.0