test: include headers for used funcs

test: include headers for used funcs

These funcs use things like wait/write/read/strcmp but sometimes
don't include the right header for them.

* test/Makefile: Add -Wall to CFLAGS.
* test/clone.c: Include unistd.h.
* test/fork.c: Include sys/wait.h.
* test/sig.c: Include unistd.h.
* test/sigkill_rain.c: Include sys/wait.h.
* test/vfork.c: Include sys/wait.h.
* test/wait_must_be_interruptible.c: Include string.h