Make now configurable so time can freeze during backburner run loop

This allow us to use fake timers, such as `sinon.useFakeTimers`, to
check the state of the app during the run loop.

For example: if a popup is supposed to be triggered in X ms from now and
it will show to the user during Y ms, then this will allow us to use
fake timers to pause between X and Y and check the state of the Ember
app at that exact time.

This is related with #263, #179 and #166.

If someone needs to have the backburner clock separate from the global
stubbed clock (as explained in #179), then `_platform.now` config should
be used to pass the native `Date.now` function.

lib/index.ts

@@ -75,7 +75,7 @@ export default class Backburner {
     platform.clearTimeout = _platform.clearTimeout || ((id) => clearTimeout(id));
     platform.next = _platform.next || ((fn) => platform.setTimeout(fn, 0));
     platform.clearNext = _platform.clearNext || platform.clearTimeout;
-    platform.now = _platform.now || Date.now;
+    platform.now = _platform.now || (() => Date.now());
 
     this._platform = platform;
 

tests/set-timeout-test.ts

@@ -50,17 +50,20 @@ QUnit.test('later', function(assert) {
   }, 20);
 });
 
-QUnit.test('later can continue when `Date.now` is monkey-patched', function(assert) {
+QUnit.test('later should rely on stubbed `Date.now`', function(assert) {
   assert.expect(1);
 
-  let arbitraryTime = +new Date();
   let bb = new Backburner(['one']);
   let done = assert.async();
+  let globalNowWasUsed = false;
 
-  Date.now = function() { return arbitraryTime; };
+  Date.now = function() {
+    globalNowWasUsed = true;
+    return originalDateNow();
+  };
 
   bb.later(() => {
-    assert.ok(true);
+    assert.ok(globalNowWasUsed);
     done();
   }, 1);
 });