Add testcase for bug 5794 (incorrect expm1 overflow)

[Roland McGrath]

> Using [BZ #N] for anything related to a bug does make sense - though you 
> need to be careful not to think someone has forgotten a NEWS entry, and so 
> make one for them, on the basis of a [BZ #N] entry, given that it may not 
> be the fix for the bug but a testcase (as here) for a bug previously 
> fixed, or only a partial fix for a bug whose fix will need several 
> patches.

We could consider amending the convention to indicate when a change should
close a bug, e.g. "[BZ #nn fixed]" or something.