areadlink, areadlink-with-size: add tests * modules/areadlink-tests: New test. * modules/areadlink-with-size-tests: Likewise. * tests/test-areadlink.h: New file. * tests/test-areadlink.c: Likewise. * tests/test-areadlink-with-size.c: Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>