[FIX] props validation: make it work through slots

A recent commit fixes the props validation code to make it work
regardless of the rendering context (important with the recent
t-call-context directive). Unfortunately, it then breaks props
validation through slots, because it assumed that the parent node in the
virtual node was the parent of the component, but it is not necessarily
true.

To fix this, we can use a simple property of the template functions:
they are bound to the current instance of the component, so we can
simply use "this"

tests/components/props_validation.test.ts

@@ -780,6 +780,32 @@ describe("props validation", () => {
     // we just check that it doesn't throw
     await expect(mount(Parent, fixture, { dev: true })).resolves.toEqual(expect.anything());
   });
+
+  test("can validate through slots", async () => {
+    class Child extends Component {
+      static props = ["message"];
+      static template = xml`<div>hey</div>`;
+    }
+
+    class Wrapper extends Component {
+      static template = xml`<t t-slot="default"/>`;
+    }
+
+    class Parent extends Component {
+      static components = { Child, Wrapper };
+      static template = xml`<Wrapper><Child /></Wrapper>`;
+    }
+
+    const app = new App(Parent, { test: true });
+    let error: OwlError | undefined;
+    const mountProm = app.mount(fixture).catch((e: Error) => (error = e));
+    await expect(nextAppError(app)).resolves.toThrow(
+      "Invalid props for component 'Child': 'message' is missing"
+    );
+    await mountProm;
+    expect(error!).toBeDefined();
+    expect(error!.message).toBe("Invalid props for component 'Child': 'message' is missing");
+  });
 });
 
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