gammainc.m: Increase tolerance on BIST test to pass on OpenBSD (bug #67370)

* gammainc.m: Increase tolerance to -2*eps from -1*eps.

Author: Rik

scripts/specfun/gammainc.m

@@ -487,7 +487,7 @@
 %!assert (gammainc ([3 2 36],[2 3 18], "upper"), ...
 %!        [4/exp(3) 5*exp(-2) (4369755579265807723 / 2977975)/exp(36)], -eps)
 %!assert (gammainc (10, 10), 1 - (5719087 / 567) * exp (-10), -eps)
-%!assert (gammainc (10, 10, "upper"), (5719087 / 567) * exp (-10), -eps)
+%!assert (gammainc (10, 10, "upper"), (5719087 / 567) * exp (-10), -2*eps)
 
 ## Test: case 8
 %!assert (gammainc (-10, 10), 3.112658265341493126871617e7, -2*eps)