nonblocking: Add tests for pipes. * tests/test-nonblocking-pipe.sh: New file. * tests/test-nonblocking-pipe-main.c: New file. * tests/test-nonblocking-pipe-child.c: New file. * tests/test-nonblocking-pipe.h: New file. * tests/test-nonblocking-writer.h: New file. * tests/test-nonblocking-reader.h: New file. * tests/test-nonblocking-misc.h: New file. * modules/nonblocking-pipe-tests: New file. * modules/nonblocking-tests (Depends-on): Add nonblocking-pipe-tests.