pin playwright to 1.8 to avoid regressions, and fix flaky tests

Playwright had some in-range breaking changes (regressions): https://github.com/microsoft/playwright/issues/10819 and https://github.com/microsoft/playwright/issues/11570

Playwright tests need to `await onceRendered()` after each route navigation to wait for render to finish before testing the state of the rendered DOM. Tests were flaky because they were looking for DOM before render finished (sometimes).

Additionally, this fixes one test that was failing only locally, but not in CI, due to a RegExp check against page.url() (not sure why it would differ on CI vs local, but now the URL is explicit).