test: use sinon mock timers for determinstic timer testing

Using sinon's ability to mock the system clock, we can make testing
the interruptable async timer more deterministc across platforms.

NODE-2684

Author: mbroadst

test/unit/utils.test.js

@@ -3,6 +3,7 @@ const eachAsync = require('../../lib/core/utils').eachAsync;
 const makeInterruptableAsyncInterval = require('../../lib/utils').makeInterruptableAsyncInterval;
 const now = require('../../lib/utils').now;
 const expect = require('chai').expect;
+const sinon = require('sinon');
 
 describe('utils', function() {
   context('eachAsync', function() {
@@ -37,7 +38,13 @@ describe('utils', function() {
   });
 
   context('makeInterruptableAsyncInterval', function() {
-    const roundToNearestMultipleOfTen = x => Math.round(x / 10) * 10;
+    before(function() {
+      this.clock = sinon.useFakeTimers();
+    });
+
+    after(function() {
+      this.clock.restore();
+    });
 
     it('should execute a method in an repeating interval', function(done) {
       let lastTime = now();
@@ -52,11 +59,13 @@ describe('utils', function() {
       );
 
       setTimeout(() => {
-        const roundedMarks = marks.map(roundToNearestMultipleOfTen);
-        expect(roundedMarks.every(mark => roundedMarks[0] === mark)).to.be.true;
+        expect(marks).to.eql([10, 10, 10, 10, 10]);
+        expect(marks.every(mark => marks[0] === mark)).to.be.true;
         executor.stop();
         done();
-      }, 50);
+      }, 51);
+
+      this.clock.tick(51);
     });
 
     it('should schedule execution sooner if requested within min interval threshold', function(done) {
@@ -75,11 +84,12 @@ describe('utils', function() {
       executor.wake();
 
       setTimeout(() => {
-        const roundedMarks = marks.map(roundToNearestMultipleOfTen);
-        expect(roundedMarks[0]).to.be.lessThan(50);
+        expect(marks).to.eql([10, 50]);
         executor.stop();
         done();
-      }, 50);
+      }, 100);
+
+      this.clock.tick(100);
     });
 
     it('should debounce multiple requests to wake the interval sooner', function(done) {
@@ -99,12 +109,12 @@ describe('utils', function() {
       }
 
       setTimeout(() => {
-        const roundedMarks = marks.map(roundToNearestMultipleOfTen);
-        expect(roundedMarks[0]).to.be.lessThan(50);
-        expect(roundedMarks.slice(1).every(mark => mark === 50)).to.be.true;
+        expect(marks).to.eql([10, 50, 50, 50, 50]);
         executor.stop();
         done();
       }, 250);
+
+      this.clock.tick(250);
     });
   });
 });