Issue mochajs#3817 prevent idle event loop during tests

lib/mocha.js

@@ -830,7 +830,25 @@ Mocha.prototype.run = function(fn) {
   exports.reporters.Base.inlineDiffs = options.useInlineDiffs;
   exports.reporters.Base.hideDiff = options.hideDiff;
 
+  // We force a timeout to exist that "squats" on the event loop till all tests
+  // complete. It does nothing except maintain a presence.
+  // See: https://github.com/mochajs/mocha/issues/3817
+  var setInterval = global.setInterval;
+  var clearInterval = global.clearInterval;
+  var eventLoopHold = setInterval(utils.noop, 1000);
+
+  function clearEventLoop() {
+    if (!eventLoopHold) {
+      return;
+    }
+    clearInterval(eventLoopHold);
+    eventLoopHold.unref();
+    eventLoopHold = null;
+  }
+  runner.once('end', clearEventLoop);
+
   function done(failures) {
+    clearEventLoop();
     fn = fn || utils.noop;
     if (reporter.done) {
       reporter.done(failures, fn);

package-scripts.js

@@ -153,6 +153,11 @@ module.exports = {
           description: 'Run Node.js reporter tests',
           hiddenFromHelp: true
         },
+        noTimeout: {
+          script: test('noTimeout', '--no-timeout "test/no-timeout/*.spec.js"'),
+          description: 'Run Node.js with no-timeout tests',
+          hiddenFromHelp: true
+        },
         only: {
           default: {
             script: `nps ${[

test/no-timeout/no-timeout.spec.js

@@ -0,0 +1,8 @@
+'use strict';
+
+describe('To be run with no timeout', function() {
+  it('should run', function(done) {
+    // note that the unref is critical for test case
+    global.setTimeout(done, 50).unref();
+  });
+});