removed unsigned int warning from src/test/agrad/matrix_test.cpp
removed unsigned int warning from src/test/agrad/matrix_test.cpp

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