test: replace concatenation with template literals

PR-URL: #14293
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: 笑斌

test/inspector/inspector-helper.js

@@ -393,7 +393,7 @@ Harness.prototype.addStderrFilter = function(regexp, callback) {
 
 Harness.prototype.assertStillAlive = function() {
   assert.strictEqual(this.running_, true,
-                     'Child died: ' + JSON.stringify(this.result_));
+                     `Child died: ${JSON.stringify(this.result_)}`);
 };
 
 Harness.prototype.run_ = function() {

test/parallel/test-https-server-keep-alive-timeout.js

@@ -12,8 +12,8 @@ const fs = require('fs');
 const tests = [];
 
 const serverOptions = {
-  key: fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem'),
-  cert: fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem')
+  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
+  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
 };
 
 function test(fn) {

test/sequential/test-debugger-debug-brk.js

@@ -4,7 +4,7 @@ common.skipIfInspectorDisabled();
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-const script = common.fixturesDir + '/empty.js';
+const script = `${common.fixturesDir}/empty.js`;
 
 function test(arg) {
   const child = spawn(process.execPath, ['--inspect', arg, script]);