Improve rounding in recent timer fix

Improve rounding in recent timer fix

* lisp/emacs-lisp/timer.el (timer-next-integral-multiple-of-time):
Use more-precise arithmetic to handle some boundary cases better
when rounding errors occur (Bug#33071).
* test/lisp/emacs-lisp/timer-tests.el:
(timer-next-integral-multiple-of-time-3):
New test, to test one of the boundary cases.
(timer-next-integral-multiple-of-time-2):
Redo so as to not assume a particular way of rounding 0.01.