Plugin E2E: Move away from waitUntil: 'networkidle'

[hugohaggmark]

With newer releases of playwright we have a need to remove the usage of waitUntil: 'networkidle' when navigating pages. This is due to the playwright team enabling the PlzDedicatedWorker feature in chromium which causes e2e test failures that look something like this:

navigating to "http://localhost:3000/XYZ", waiting until "networkidle"

networkidle is the default used by plugin-e2e in the GrafanaPage here model when going to a page. However the docs related to page.goto() discourage it's usage:

DISCOURAGED consider operation to be finished when there are no network connections for at least 500 ms. Don't use this method for testing, rely on web assertions to assess readiness instead.

See #1728