build: Include sys/wait.h in commandtest.c

Commit 631923e7 used a few macros from sys/wait.h without including
it. On Linux, they were also defined in stdlib.h, but on FreeBSD
the build failed:

../../tests/commandtest.c: In function 'test1':
warning: implicit declaration of function 'WIFEXITED'
warning: nested extern declaration of 'WIFEXITED' [-Wnested-externs]

tests/commandtest.c

@@ -26,6 +26,7 @@
 #include <unistd.h>
 #include <signal.h>
 #include <sys/stat.h>
+#include <sys/wait.h>
 #include <fcntl.h>
 
 #include "testutils.h"