Add tests for math.mk

Turn commented out tests into assertion tests, and add more for
functions that didn't have tests.

Test: make -f ${ANDROID_BUILD_TOP}/build/make/core/math.mk RUN_MATH_TESTS=true
Test: $(get_build_var CKATI) -f ${ANDROID_BUILD_TOP}//build/make/core/math.mk RUN_MATH_TESTS=true
Change-Id: I411844fbf00268f863260b7ee42d1795f05c0d16