Adjust new test case for more portability.

Adjust new test case for more portability.

Early returns from the buildfarm say that most critters are good with
commit cbdb8b4c0, but gaur gives unexpected results with the test case
involving a float8 that's one-ULP-less-than-2^63.  It appears that that
platform's version of rint() rounds that value up to 2^63 instead of
leaving it be.  This is possibly a bug, and it's also possible that no
other platform anybody is using anywhere behaves likewise.  Still, the
point of the test is not to insist that everybody's rint() behaves exactly
the same.  Let's use two-ULPs-less-than-2^63 instead, which I've tested
to act the same on gaur as on more modern hardware.

(This is, more or less, exactly the portability issue I'd feared might
arise...)

Discussion: https://postgr.es/m/15519-4fc785b483201ff1@postgresql.org