unix: Use fork instead of vfork in runner.

test/runner-unix.c

@@ -80,7 +80,7 @@ int process_start(char* name, process_info_t* p) {
   p->terminated = 0;
   p->status = 0;
 
-  pid_t pid = vfork();
+  pid_t pid = fork();
 
   if (pid < 0) {
     perror("vfork");