test: refactor inspector-helper.js

There are two instances of `setTimeout()` called without a duration in
`inspector-helper.js`. Change to `setImmediate()` for clarity that it
isn't a mistake.

PR-URL: #9499
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/inspector/inspector-helper.js

@@ -259,7 +259,7 @@ TestSession.prototype.expectStderrOutput = function(regexp) {
 
 TestSession.prototype.runNext_ = function() {
   if (this.task_) {
-    setTimeout(() => {
+    setImmediate(() => {
       this.task_(() => {
         this.task_ = this.task_.next_;
         this.runNext_();
@@ -338,7 +338,7 @@ Harness.prototype.addStderrFilter = function(regexp, callback) {
 };
 
 Harness.prototype.run_ = function() {
-  setTimeout(() => {
+  setImmediate(() => {
     this.task_(() => {
       this.task_ = this.task_.next_;
       if (this.task_)