stdlib tests: Avoid code duplication. * modules/stdlib-tests (Files): Add tests/test-sys_wait.h. * modules/sys_wait-tests (Files): Likewise. * tests/test-sys_wait.h: New file, extracted from tests/test-stdlib.c. * tests/test-stdlib.c: Include test-sys_wait.h. (main): Invoke test_sys_wait_macros. * tests/test-sys_wait.c: Include test-sys_wait.h. (main): Invoke test_sys_wait_macros.