[AIRFLOW-3885] ~20x speed-up of slowest unit test (#4726)
[AIRFLOW-3885] ~20x speed-up of slowest unit test (#4726)

The test `SchedulerJobTest.test_scheduler_start_date` is the slowest test,
taking ~5 minutes on average:

    >>> [success] 12.99% tests.test_jobs.SchedulerJobTest.test_scheduler_start_date: 295.1935s
	[success] 6.79% tests.test_jobs.SchedulerJobTest.test_scheduler_multiprocessing: 154.2304s
	[success] 6.72% tests.test_jobs.SchedulerJobTest.test_scheduler_task_start_date: 152.7215s
	[success] 4.34% tests.test_jobs.SchedulerJobTest.test_new_import_error_replaces_old: 98.7339s
	[success] 3.63% tests.test_jobs.SchedulerJobTest.test_remove_error_clears_import_error: 82.4062s

After setting the subdirectory and eliminating (I think) redundant scheduler
loops, the test time comes down to ~15 seconds.
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow