getlogin tests: Avoid #ifdefs when sharing code between modules. * modules/getlogin_r-tests (Files): Add tests/test-getlogin.h. * modules/getlogin-tests (Files): Likewise. Remove tests/test-getlogin_r.c. * tests/test-getlogin.h: Extracted from tests/test-getlogin_r.c. * tests/test-getlogin.c: Extracted from tests/test-getlogin_r.c. * tests/test-getlogin_r.c: Include test-getlogin.h. Omit code that tests getlogin().