Fix double-free introduced by leak plug

Once all instances of a given test are started all of the remaining
tests would free the test data structure, we needed another counter
to keep track of ongoing tests so that only the last one to finish
would do the single free.

bin/varnishtest/vtc_main.c

@@ -72,6 +72,7 @@ struct vtc_tst {
 	const char		*filename;
 	char			*script;
 	unsigned		ntodo;
+	unsigned		nwait;
 };
 
 struct vtc_job {
@@ -370,7 +371,8 @@ tst_cb(const struct vev *ve, int what)
 			VEV_Stop(vb, jp->evt);
 			free(jp->evt);
 		}
-		if (jp->tst->ntodo == 0) {
+		jp->tst->nwait--;
+		if (jp->tst->nwait == 0) {
 			free(jp->tst->script);
 			FREE_OBJ(jp->tst);
 		}
@@ -644,6 +646,7 @@ read_file(const char *fn, int ntest)
 	tp->filename = fn;
 	tp->script = p;
 	tp->ntodo = ntest;
+	tp->nwait = ntest;
 	VTAILQ_INSERT_TAIL(&tst_head, tp, list);
 	return (0);
 }