Test coverage gap in NudgeToCalendarUnit

[catamorphism]

codecov reports the following code (currently lines 3313-3330 of ecmascript.mjs) as a coverage gap:

else if (sign == -1) {
    if (!(nudgeWindow.endEpochNs.leq(destEpochNs) && destEpochNs.leq(nudgeWindow.startEpochNs))) {
      // Retry nudge window if it's out of bounds
      nudgeWindow = ComputeNudgeWindow(
        sign,
        duration,
        originEpochNs,
        isoDateTime,
        timeZone,
        calendar,
        increment,
        unit,
        true
      );
      assert(
        nudgeWindow.endEpochNs.leq(destEpochNs) && destEpochNs.leq(nudgeWindow.startEpochNs),
        `${unit} was 0 days long`
      );
      didExpandCalendarUnit = true;

and indeed, we don't have tests that exercise the case where the duration is negative and we need to retry the nudge window.

I haven't been able to construct an example to exercise this code. In the (sign == 1) case, the nudge window may need to be retried if (for example) the duration being added could be expressed as either 1 month, 10 hours or 29 days, 10 hours (such as when the relativeTo date is 2020-01-31). The same case doesn't seem to happen going backwards as the duration is unambiguously a month.

However, I haven't convinced myself that this is always true.