test: clarify role of domains in test

Add a comment to clarify how the tests work and their purpose.

Also removes unnecessary assignment of domain module to a variable.

PR-URL: #4474
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Julien Gilli <jgilli@fastmail.fm>

test/parallel/test-microtask-queue-integration-domain.js

@@ -1,7 +1,13 @@
 'use strict';
 require('../common');
 var assert = require('assert');
-var domain = require('domain');
+
+// Requiring the domain module here changes the function that is used by node to
+// call process.nextTick's callbacks to a variant that specifically handles
+// domains. We want to test this specific variant in this test, and so even if
+// the domain module is not used, this require call is needed and must not be
+// removed.
+require('domain');
 
 var implementations = [
   function(fn) {

test/parallel/test-microtask-queue-run-domain.js

@@ -1,7 +1,13 @@
 'use strict';
 require('../common');
 var assert = require('assert');
-var domain = require('domain');
+
+// Requiring the domain module here changes the function that is used by node to
+// call process.nextTick's callbacks to a variant that specifically handles
+// domains. We want to test this specific variant in this test, and so even if
+// the domain module is not used, this require call is needed and must not be
+// removed.
+require('domain');
 
 function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);

test/parallel/test-microtask-queue-run-immediate-domain.js

@@ -1,7 +1,13 @@
 'use strict';
 require('../common');
 var assert = require('assert');
-var domain = require('domain');
+
+// Requiring the domain module here changes the function that is used by node to
+// call process.nextTick's callbacks to a variant that specifically handles
+// domains. We want to test this specific variant in this test, and so even if
+// the domain module is not used, this require call is needed and must not be
+// removed.
+require('domain');
 
 function enqueueMicrotask(fn) {
   Promise.resolve().then(fn);