test: Add a test case for vec growth

src/test/run-pass/vec-growth.rs

@@ -0,0 +1,15 @@
+// xfail-stage0
+
+fn main() {
+    auto v = vec(1);
+    v += vec(2);
+    v += vec(3);
+    v += vec(4);
+    v += vec(5);
+    check (v.(0) == 1);
+    check (v.(1) == 2);
+    check (v.(2) == 3);
+    check (v.(3) == 4);
+    check (v.(4) == 5);
+}
+