tests: test more substitute headers

tests: test more substitute headers

Add tests of common headers, in preparation of always providing
a gnulib wrapper (for improved LINK_WARNING support).

* modules/ctype-tests: New file.
* modules/dirent-tests: Likewise.
* modules/spawn-tests: Likewise.
* modules/sys_file-tests: Likewise.
* modules/sys_ioctl-tests: Likewise.
* modules/sys_wait-tests: Likewise.
* tests/test-ctype.c: Likewise.
* tests/test-dirent.c: Likewise.
* tests/test-spawn.c: Likewise.
* tests/test-sys_file.c: Likewise.
* tests/test-sys_ioctl.c: Likewise.
* tests/test-sys_wait.c: Likewise.
* m4/spawn_h.m4 (gl_SPAWN_H): Replace header if it is missing.
* lib/sys_file.in.h (LOCK_SH, LOCK_EX, LOCK_UN, LOCK_NB): Provide
whether or not flock is in use.

Signed-off-by: Eric Blake <ebb9@byu.net>