Remove use of deprecated functions from test code

lib/input.js

@@ -60,7 +60,9 @@ const _createInputDescriptor = function _createInputDescriptor (input, inputOpti
  * @param {Function} callback
  */
 const _write = function _write (chunk, encoding, callback) {
+  /* istanbul ignore else */
   if (Array.isArray(this.options.input.buffer)) {
+    /* istanbul ignore else */
     if (is.buffer(chunk)) {
       this.options.input.buffer.push(chunk);
       callback();