arrow

GH-44915: [C++] Add WithinUlp testing functions (#44906)

### Rationale for this change

When testing math-related functions, we might want to check that some results are very close to an expected value, but not necessarily exactly equal.

### What changes are included in this PR?

Add functions that test whether two floating-point values are within N ulps.

("ulp" stands for "unit in the last place": https://en.wikipedia.org/wiki/Unit_in_the_last_place)

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Potentially more useful error messages.

* GitHub Issue: #44915

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>