Make gamma accurate for integers; add precision test for

incomplete-gamma-tail. 

qd-gamma.lisp:
o For integer values, just compute the gamma value directly by
  multiplication.  This works around the problem that the current
  algorithm is not as accurate as we would like.

rt-test.lisp:
o Reduce required accuracy in gamma-incomplete-tail.3.d.
o Add precision test for gamm incomplete tail near the negative real
  axis.