test: refactor test-readline-keys

* replace `util._extend()` with `Object.assign()`
* extract repeated map function to a single instance
* remove unneeded truthiness-check ternary on Objects

Backport-PR-URL: #16947
PR-URL: #11281
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-readline-keys.js

@@ -3,7 +3,6 @@ const common = require('../common');
 const PassThrough = require('stream').PassThrough;
 const assert = require('assert');
 const inherits = require('util').inherits;
-const extend = require('util')._extend;
 const Interface = require('readline').Interface;
 
 
@@ -12,6 +11,10 @@ function FakeInput() {
 }
 inherits(FakeInput, PassThrough);
 
+function extend(k) {
+  return Object.assign({ ctrl: false, meta: false, shift: false }, k);
+}
+
 
 const fi = new FakeInput();
 const fo = new FakeInput();
@@ -32,9 +35,7 @@ function addTest(sequences, expectedKeys) {
     expectedKeys = [ expectedKeys ];
   }
 
-  expectedKeys = expectedKeys.map(function(k) {
-    return k ? extend({ ctrl: false, meta: false, shift: false }, k) : k;
-  });
+  expectedKeys = expectedKeys.map(extend);
 
   keys = [];
 
@@ -65,9 +66,7 @@ const addKeyIntervalTest = (sequences, expectedKeys, interval = 550,
       expectedKeys = [ expectedKeys ];
     }
 
-    expectedKeys = expectedKeys.map(function(k) {
-      return k ? extend({ ctrl: false, meta: false, shift: false }, k) : k;
-    });
+    expectedKeys = expectedKeys.map(extend);
 
     const keys = [];
     fi.on('keypress', (s, k) => keys.push(k));