test: Wait for a page load in Browser.open()

[martinpitt]

Commit 8284618 dropped all explicit "wait for page load" calls (fka. expect_load()) in favor of allowing page loads during wait commands. This is better in almost all cases, but it created a race condition on the login page in tests which logged out and back in. In particular, this can happen:

1. Test calls Browser.logout().

2. That destroys a lot of execution contexts from the session, and creates a new one for the login page. It waits until the #login field becomes visible.

3. Test starts a new session by calling Browser.open() usually via Browser.login_and_go(). This results in a Page.navigate() CDP call. Sometimes this takes a while.

4. Browser.try_login() fills in the login form. In most cases, the wait: ph_is_present("#login") command runs into the page load from 3., sees the destroyed/new execution context, and resumes on the new frame.

   However, if cockpit/ws or the browser take a while to load the login page, it could happen that it gets all the way to e.g. filling in the user or even password form before the page load from 3. happens. That page load resets the form.

5. try_login() clicks on "Login" button, which fails because no user or password is given.

Fix this by always waiting for a page load in Browser.open() after a Page.navigate(). But we don't need the unnecessary complex and brittle polling machinery of the old expectLoad() before commit 8284618. We can use the loadEventFired CDP signal, which happens after the page and all of its frames finished loading, and execution contexts are set up. Introduce a new little waitPageLoad() helper which re-uses the existing pageLoadHandler promise.

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This fixes this common flake pair, which both fail in > 30% of runs.

See #19578 for the debugging note details and a stress test with amplification.

• Wait for stress-test to succeed in debug Test{AD,IPA}.testQualifiedUsers #19578.

[martinpitt]

Meh, too soon -- several test failures which now hang forever. So at the very least, we need to teach this thing about a timeout. TestKeys.testAuthorizedKeys reproduces fine locally.