Move TestRuntimeCache into non-test file

pkg/kubelet/container/export_test.go → pkg/kubelet/container/runtime_cache_fake.go

@@ -16,8 +16,10 @@ limitations under the License.
 
 package container
 
-// TestRunTimeCache embeds runtimeCache with some additional methods for
-// testing.
+// TestRunTimeCache embeds runtimeCache with some additional methods for testing.
+// It must be declared in the container package to have visibility to runtimeCache.
+// It cannot be in a "..._test.go" file in order for runtime_cache_test.go to have cross-package visibility to it.
+// (cross-package declarations in test files cannot be used from dot imports if this package is vendored)
 type TestRuntimeCache struct {
 	runtimeCache
 }