Allow run() to run at arbitrary time

Currently when run() is called it uses the current time as the run
timestamp. However it would be useful to run the scheduler at a
specific time. So allow an optional param with a DateTime to run().

Reasons for running at a specific time other than now() could be:

- Catching up on 'missed' runs, eg. when the system was down.

- Making sure the scheduler runs at an intended timestamp eg. when
cron is started at midnight it could be slow and start the php process
only at 00:01 and thus missing all midnight schedules.

- Improve testability of the scheduler (fake the time)

src/GO/Scheduler.php

@@ -149,13 +149,16 @@ public function raw($command, $args = [], $id = null)
     /**
      * Run the scheduler.
      *
+     * @param DateTime $runTime  Optional, run at specific moment
      * @return array  Executed jobs
      */
-    public function run()
+    public function run($runTime = null)
     {
         $jobs = $this->getQueuedJobs();
 
-        $runTime = new DateTime('now');
+        if (is_null($runTime)) {
+            $runTime = new DateTime('now');
+        }
 
         foreach ($jobs as $job) {
             if ($job->isDue($runTime)) {

tests/GO/SchedulerTest.php

@@ -1,5 +1,6 @@
 <?php namespace GO\Job\Tests;
 
+use DateTime;
 use GO\Scheduler;
 use PHPUnit\Framework\TestCase;
 
@@ -304,4 +305,18 @@ public function testShouldRunDelayedJobsIfDueWhenCreated()
 
         $this->assertEquals(2, count($executed));
     }
+
+    public function testShouldRunAtSpecificTime()
+    {
+        $scheduler = new Scheduler();
+        $runTime = new DateTime('2017-09-13 00:00:00');
+
+        $scheduler->call(function () {
+            // do nothing
+        })->daily('00:00');
+
+        $executed = $scheduler->run($runTime);
+
+        $this->assertEquals(1, count($executed));
+    }
 }