libc-test: add libc testsuite for musl

libc-test is a collection of unit test to measure the correctness and
robustness of a C/POSIX standard library implementation. It is developed
as part of the musl project.

Signed-off-by: Chase Qi <chase.qi@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>