Remove redundant self._next_triggers construction and add AndTrigger test.

[bmeares]

Changes

Fixes #911.

When combining triggers (IntervalTrigger, CronTrigger, CalendarIntervalTrigger) with the AndTrigger, this patch ensures the expected datetimes are fired. The now-fixed bug was that every other interval was skipped in certain scenarios, such as when using IntervalTrigger with days.

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* Fix big bad boo-boo in the async scheduler (#999 <https://github.com/agronholm/apscheduler/issues/999>_; PR by Yourname)

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[agronholm]

Nice catch with the redundant line. I just have some comments on the tests.

[agronholm]

Would you mind removing the rest of the redundant zeroes from the datetime() calls? Then I'm ready to merge this.

[bmeares]

All cleaned up, ready to merge👍🏻

[agronholm]

Thanks!