[FIX] app: sync destroy everythig when app is destroyed

Before this commit, the App destroy method would not destroy everything
synchronously. The code scheduled a flush, which is asynchronous, to
finally clean up the task lists, and destroy the cancelled nodes. But
this is not really good for tests: we need a stronger guarantee that
nothing will happen after the destroy.

With this commit, we simply call the processTask method immediately
after destroying the root component, so all cancelled nodes will be
destroyed immediately.

tests/app/__snapshots__/app.test.ts.snap

@@ -15,6 +15,34 @@ exports[`app App supports env with getters/setters 1`] = `
 }"
 `;
 
+exports[`app app: clear scheduler tasks and destroy cancelled nodes immediately on destroy 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  const comp1 = app.createComponent(\`B\`, true, false, false, []);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2,b3;
+    b2 = text(\`A\`);
+    if (ctx['state'].value) {
+      b3 = comp1({}, key + \`__1\`, node, this, null);
+    }
+    return multi([b2, b3]);
+  }
+}"
+`;
+
+exports[`app app: clear scheduler tasks and destroy cancelled nodes immediately on destroy 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return text(\`B\`);
+  }
+}"
+`;
+
 exports[`app can configure an app with props 1`] = `
 "function anonymous(app, bdom, helpers
 ) {

tests/app/app.test.ts

@@ -1,6 +1,14 @@
-import { App, Component, mount, xml } from "../../src";
+import { App, Component, mount, onWillStart, useState, xml } from "../../src";
 import { status } from "../../src/runtime/status";
-import { makeTestFixture, snapshotEverything, nextTick, elem } from "../helpers";
+import {
+  makeTestFixture,
+  snapshotEverything,
+  nextTick,
+  elem,
+  useLogLifecycle,
+  makeDeferred,
+  nextMicroTick,
+} from "../helpers";
 
 let fixture: HTMLElement;
 
@@ -94,4 +102,44 @@ describe("app", () => {
     expect(iframeDoc.contains(div)).toBe(false);
     expect(status(comp)).toBe("destroyed");
   });
+
+  test("app: clear scheduler tasks and destroy cancelled nodes immediately on destroy", async () => {
+    let def = makeDeferred();
+    class B extends Component {
+      static template = xml`B`;
+      setup() {
+        useLogLifecycle();
+        onWillStart(() => def);
+      }
+    }
+    class A extends Component {
+      static template = xml`A<t t-if="state.value"><B/></t>`;
+      static components = { B };
+      state = useState({ value: false });
+      setup() {
+        useLogLifecycle();
+      }
+    }
+
+    const app = new App(A);
+    const comp = await app.mount(fixture);
+    expect(["A:setup", "A:willStart", "A:willRender", "A:rendered", "A:mounted"]).toBeLogged();
+
+    comp.state.value = true;
+    await nextTick();
+    expect(["A:willRender", "B:setup", "B:willStart", "A:rendered"]).toBeLogged();
+
+    // rerender to force the instantiation of a new B component (and cancelling the first)
+    comp.render();
+    await nextMicroTick();
+    expect(["A:willRender", "B:setup", "B:willStart", "A:rendered"]).toBeLogged();
+
+    app.destroy();
+    expect([
+      "A:willUnmount",
+      "B:willDestroy",
+      "A:willDestroy",
+      "B:willDestroy", // make sure the 2 B instances have been destroyed synchronously
+    ]).toBeLogged();
+  });
 });