[FIX] qweb: re-add test on memory leak

Re-add test from 6e185f9.

tests/qweb/__snapshots__/qweb_memory.test.ts.snap

@@ -0,0 +1,24 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`memory t-foreach does not leak stuff in global scope 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component } = bdom;
+  let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue } = helpers;
+  
+  let block1 = createBlock(\`<p><block-child-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const [k_block2, v_block2, l_block2, c_block2] = prepareList([3,2,1]);
+    for (let i1 = 0; i1 < l_block2; i1++) {
+      ctx[\`item\`] = v_block2[i1];
+      ctx[\`item_index\`] = i1;
+      let key1 = ctx['item_index'];
+      c_block2[i1] = withKey(text(ctx['item']), key1);
+    }
+    let b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;