spawn-pipe: Fix test failure when running under QEMU user-mode. * tests/test-spawn-pipe-child.c: Include <stdbool.h>, <string.h>, qemu.h. (main): Under QEMU user-mode, allow fd 2 or fd 3 to be open. * modules/spawn-pipe-tests (Files): Add qemu.h. (Depends-on): Add stdbool.