read() and write() return ssize_t, not size_t.

rv < 0 error checks were always false.

test/runner-unix.c

@@ -248,7 +248,7 @@ int process_copy_output(process_info_t *p, int fd) {
     return -1;
   }
 
-  size_t nread, nwritten;
+  ssize_t nread, nwritten;
   char buf[1024];
 
   while ((nread = read(fileno(p->stdout_file), buf, 1024)) > 0) {