gh-88352: Make TimedRotatingFileHandler tests more stable (GH-116409)

The tests failed (with less than 1% probability) if for example the file
was created at 11:46:03.999, but the record was emitted at 11:46:04.001,
with atTime=11:46:04, which caused an unexpected rollover. Ensure that the
tests are always run within the range of the same whole second.

Also share code between test_rollover_at_midnight and test_rollover_at_weekday.