[FIX] app: validate props for root component in dev mode

Before this commit, only sub components were validated.

src/app/app.ts

@@ -60,6 +60,9 @@ export class App<
 
   mount(target: HTMLElement, options?: MountOptions): Promise<Component<P, E> & InstanceType<T>> {
     App.validateTarget(target);
+    if (this.dev) {
+      this.helpers.validateProps(this.Root, this.props);
+    }
     const node = this.makeNode(this.Root, this.props);
     const prom = this.mountNode(node, target, options);
     this.root = node;

tests/components/props_validation.test.ts

@@ -87,6 +87,22 @@ describe("props validation", () => {
     expect(error!.message).toBe("Invalid props for component 'SubComp': 'message' is missing");
   });
 
+  test("validate props for root component", async () => {
+    class Root extends Component {
+      static props = ["message"];
+      static template = xml`<div t-esc="message"/>`;
+    }
+
+    let error: Error;
+    try {
+      await mount(Root, fixture, { dev: true });
+    } catch (e) {
+      error = e as Error;
+    }
+    expect(error!).toBeDefined();
+    expect(error!.message).toBe("Invalid props for component 'Root': 'message' is missing");
+  });
+
   test("validate simple types", async () => {
     const Tests = [
       { type: Number, ok: 1, ko: "1" },