[REF] qweb: use global nextID

and reset it to 1 before each test s.t. snapshots are deterministic.

src/qweb/extensions.ts

@@ -201,7 +201,7 @@ QWeb.addDirective({
     if (!ctx.parentNode) {
       ctx.rootContext.shouldDefineResult = true;
       ctx.rootContext.shouldDefineUtils = true;
-      parentNode = `children${ctx.nextID++}`;
+      parentNode = `children${ctx.generateID()}`;
       ctx.addLine(`let ${parentNode}= []`);
       ctx.addLine(`result = {}`);
     }