The valid range for temporal values (`nsMinInstant`/`nsMaxInstant`)
means performing nanosecond-valued integers could lead to an overflow.
NB: Only the `roundingMode: "day"` case was affected, as all others were
already performing the division on floating-point `fractional_second`
values. I'm adding `.0` suffixes everywhere to make this fact clearer.
This adds a few local tests as well, as those are tested with sanitizers
enabled by default, unlike test262.