Separate quiet NaN from infinity and expand the nan-with-payload test
to a general test that bits outside the exponent don't break qIsNan().
Generally test more thoroughly and systematically.
Tests for signalling NaN shall follow.
Change-Id: Ib35dabacc8ebcc9a0761df38f6f419f0398d0e20
Reviewed-by: Erik Verbruggen <erik.verbruggen@me.com>