test: fix flaky partials opt-out tests by adding waitForNavigation()

[SisyphusZheng]

Two tests in partials_test.tsx were marked ignore: true due to flakiness.

Root Cause

Both tests click links with f-client-nav={false}, triggering full-page navigations. Neither waited for the navigation to complete:

// Broken (flaky) — click races with navigation
await page.locator(.update).click();
await page.locator(.done).wait();

The third test (opt out of partial navigation in island) uses the correct pattern and is NOT flaky:

// Correct — navigation is awaited
await Promise.all([
    page.waitForNavigation(),
    page.locator(.update).click(),
]);

Fix

Apply the same Promise.all([waitForNavigation(), click]) pattern to both flaky tests and remove the ignore: true markers.

Closes #3858