removed unsigned int warnings for loops in test/prob/transform
removed unsigned int warnings for loops in test/prob/transform

stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math