chore: fix async dialog specs when they fail (#5859)

When this test was failing, it would cause no future tests to run. This
was because the `expect` call within the `page.on` callback would throw
an error, and that would trigger a unhandled promise rejection that
caused the test framework to stop.

The fundamental issue here is making `expect` calls within callbacks.
They are brittle due to the fact that they throw, and the test framework
won't catch it, but also because you have no guarantee that they will
run. If the callback is never executed you dont' know about it.

Although it's slightly more code, using a stub is the way to do this.
Not only can we assert that the stub was called, we can make synchronous
`expect` calls that Mocha will pick up properly if they fail.

Before this change, running the tests (and making it fail on purpose)
would cause all test execution to stop:

```
> puppeteer@3.0.4-post unit /Users/jacktfranklin/src/puppeteer
> mocha --config mocha-config/puppeteer-unit-tests.js

  .(node:69580) UnhandledPromiseRejectionWarning: Error: expect(received).toBe(expected) // Object.is equality

Expected: "yes."
Received: ""
    at Page.<anonymous> (/Users/jacktfranklin/src/puppeteer/test/dialog.spec.js:42:37)
    [snip]
(node:69580) UnhandledPromiseRejectionWarning: Unhandled promise rejection ... [snip]
```

But with this change, the rest of the tests run:

```
> puppeteer@3.0.4-post unit /Users/jacktfranklin/src/puppeteer
> mocha --config mocha-config/puppeteer-unit-tests.js

  Page.Events.Dialog
    ✓ should fire
    1) should allow accepting prompts
    ✓ should dismiss the prompt

  2 passing (2s)
  1 failing

  1) Page.Events.Dialog
       should allow accepting prompts:
     Error: expect(received).toBe(expected) // Object.is equality

Expected: "yes."
Received: ""
      at Context.<anonymous> (test/dialog.spec.js:53:35)
      at processTicksAndRejections (internal/process/task_queues.js:94:5)
```

This is much better because one failing test now doesn't stop the rest
of the test suite.

This probably isn't the only instance of this in the codebase so I
propose as we encounter them we fix them usng this commit as the
template.

test/dialog.spec.js

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 const expect = require('expect');
+const sinon = require('sinon');
+
 const {
   getTestState,
   setupTestPageAndContextHooks,
@@ -34,16 +36,23 @@ describe('Page.Events.Dialog', function () {
     });
     await page.evaluate(() => alert('yo'));
   });
+
   itFailsFirefox('should allow accepting prompts', async () => {
     const { page } = getTestState();
 
-    page.on('dialog', (dialog) => {
-      expect(dialog.type()).toBe('prompt');
-      expect(dialog.defaultValue()).toBe('yes.');
-      expect(dialog.message()).toBe('question?');
+    const onDialog = sinon.stub().callsFake((dialog) => {
       dialog.accept('answer!');
     });
+    page.on('dialog', onDialog);
+
     const result = await page.evaluate(() => prompt('question?', 'yes.'));
+
+    expect(onDialog.callCount).toEqual(1);
+    const dialog = onDialog.firstCall.args[0];
+    expect(dialog.type()).toBe('prompt');
+    expect(dialog.defaultValue()).toBe('yes.');
+    expect(dialog.message()).toBe('question?');
+
     expect(result).toBe('answer!');
   });
   it('should dismiss the prompt', async () => {