test: Add missing break in stress-test.c

Found by coverity:

https://bugzilla.redhat.com/show_bug.cgi?id=756069
diff --git a/test/stress-test.c b/test/stress-test.c
index 0b48da3..7fc067e 100644
--- a/test/stress-test.c
+++ b/test/stress-test.c
@@ -412,6 +412,7 @@
 
 	case 6: case 7:
 	    n_rects = 3;
+	    break;
 
 	default:
 	    n_rects = lcg_rand_n (100);