[BUG] - Stop func might not wait for job to finish

[hiroebe]

Describe the bug

(*Scheduler).Stop func should wait for all jobs to finish due to its doc, but it possibly returns before all jobs finished.

To Reproduce

(This is a concurrent issue and a bit hard to reproduce)

1. Add time.Sleep(time.Second) after the lines below (to raise the concurrent issue on purpose)
   

   gocron/executor.go

   Lines 47 to 48 in e315112

   	case f := <-e.jobFunctions:
   	runningJobsWg.Add(1)

2. Write some code like this:

func main() {
	s := gocron.NewScheduler(time.UTC)
	s.Every(1).Second().Do(func() {
		fmt.Println("Job started")
		time.Sleep(5 * time.Second)
		fmt.Println("Job finished")
	})
	go func() {
		time.Sleep(3 * time.Second)
		fmt.Println("Trying to stop")
		s.Stop()
		fmt.Println("Stopped")
	}()
	s.StartAsync()

	time.Sleep(10 * time.Second)
}

The output on my machine is:

Job started
Job started
Trying to stop
Stopped
Job started
Job finished
Job finished
Job finished

As we can see, Stop() func returns before the jobs finished. Expected behavior is that Stopped log is shown at the end.

Version

latest main: e315112

Expected behavior

Stop() func always waits for the jobs to finish.

Additional context

N/A