Add specs for intervals that "clear themselves"

Add specs to test if issue #655 is present: the handler of an interval
cannot successfully clear the same interval that generated it's
invocation.

The most direct test consist in setting an interval with a handler that
calls clearInterval over that same interval and make the clock tick for
double of it's period. If the issue is present the interval's handler
will be called twice. If the issue is not present, the first invocation
of the handler will avoid a second one (because of the clearInterval).

Another test is included in order to check if recurring scheduled
functions are rescheduled before being called. Doing this in the reverse
order is the exact cause of the issue.