fix(runtime-core): allow spying on proxy methods (#4216)

Since Jest v26.6.1, the mock method changed (see this commit 30e8020362)  to rely on `Object.defineProperty` in some cases.

This breaks spying on proxy's methods, because even if Jest is properly calling `Object.defineProperty`, the cached value in the `get` section of the proxy is never updated, and the spy is in fact never used.
This is easily reproducible as vue-next already uses a version of jest with these changes.

This is blocking projects (like vue-test-utils-next and vue-cli) to update to recent Jest versions.

This commit adds a `defineProperty` method to the proxy handler, that properly updates the defined value in the cache.

packages/runtime-core/__tests__/componentPublicInstance.spec.ts

@@ -214,6 +214,74 @@ describe('component: proxy', () => {
     ])
   })
 
+  test('allow updating proxy with Object.defineProperty', () => {
+    let instanceProxy: any
+    const Comp = {
+      render() {},
+      setup() {
+        return {
+          isDisplayed: true
+        }
+      },
+      mounted() {
+        instanceProxy = this
+      }
+    }
+
+    const app = createApp(Comp)
+
+    app.mount(nodeOps.createElement('div'))
+
+    Object.defineProperty(instanceProxy, 'isDisplayed', { value: false })
+
+    expect(instanceProxy.isDisplayed).toBe(false)
+
+    Object.defineProperty(instanceProxy, 'isDisplayed', { value: true })
+
+    expect(instanceProxy.isDisplayed).toBe(true)
+
+    Object.defineProperty(instanceProxy, 'isDisplayed', {
+      get() {
+        return false
+      }
+    })
+
+    expect(instanceProxy.isDisplayed).toBe(false)
+
+    Object.defineProperty(instanceProxy, 'isDisplayed', {
+      get() {
+        return true
+      }
+    })
+
+    expect(instanceProxy.isDisplayed).toBe(true)
+  })
+
+  test('allow spying on proxy methods', () => {
+    let instanceProxy: any
+    const Comp = {
+      render() {},
+      setup() {
+        return {
+          toggle() {}
+        }
+      },
+      mounted() {
+        instanceProxy = this
+      }
+    }
+
+    const app = createApp(Comp)
+
+    app.mount(nodeOps.createElement('div'))
+
+    const spy = jest.spyOn(instanceProxy, 'toggle')
+
+    instanceProxy.toggle()
+
+    expect(spy).toHaveBeenCalled()
+  })
+
   // #864
   test('should not warn declared but absent props', () => {
     const Comp = {