1c85849 "fixed"
test-domain-exit-dispose-again by changing its logic to test that
process.domain was cleared properly in case an error was thrown from a
timer's callback.

However, it became clear when reviewing a recent change that refactors
lib/timers.js that it was not quite the intention of the original test.
Thus, this change adds the original implementation of
test-domain-exit-dispose-again back, with comments that make its
implementation easier to understand.

It also preserve the changes made by
1c85849, but it moves them to a new
test file named test-timers-reset-process-domain-on-throw.js.