e2e storage: add compile test for public TestSuite API

This will catch accidentally adding a new interface function which
isn't exported. For example, an attempt to implement a new unexported
"foobar()" function then leads to:

test/e2e/storage/testsuites/api_test.go:54:5: cannot use &fakeSuite literal (type *fakeSuite) as type testsuites.TestSuite in assignment:
	*fakeSuite does not implement testsuites.TestSuite (missing testsuites.foobar method)
		have foobar()
		want testsuites.foobar()
FAIL	k8s.io/kubernetes/test/e2e/storage/testsuites [build failed]

test/e2e/storage/testsuites/api_test.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2019 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 testsuites_test is used intentionally to ensure that the
+// code below only has access to exported names. It doesn't have any
+// actual test. That the custom volume test suite defined below
+// compile is the test.
+//
+// It's needed because we don't have any in-tree volume test
+// suite implementations that aren't in the "testuites" package itself.
+// We don't need this for the "TestDriver" interface because there
+// we have implementations in a separate package.
+package testsuites_test
+
+import (
+	"k8s.io/kubernetes/test/e2e/framework/volume"
+	"k8s.io/kubernetes/test/e2e/storage/testpatterns"
+	"k8s.io/kubernetes/test/e2e/storage/testsuites"
+)
+
+type fakeSuite struct {
+}
+
+func (f *fakeSuite) GetTestSuiteInfo() testsuites.TestSuiteInfo {
+	return testsuites.TestSuiteInfo{
+		Name:               "fake",
+		FeatureTag:         "",
+		TestPatterns:       []testpatterns.TestPattern{testpatterns.DefaultFsDynamicPV},
+		SupportedSizeRange: volume.SizeRange{Min: "1Mi", Max: "1Gi"},
+	}
+}
+
+func (f *fakeSuite) DefineTests(testsuites.TestDriver, testpatterns.TestPattern) {
+}
+
+func (f *fakeSuite) SkipRedundantSuite(testsuites.TestDriver, testpatterns.TestPattern) {
+}
+
+var _ testsuites.TestSuite = &fakeSuite{}