Update test to support new frontend UI

[rgaiacs]

BinderHub frontend UI migrated to vanilla HTML to React based app in jupyterhub/binderhub#1856. Because of this, the test

mybinder.org-deploy/tests/test_federation_redirect.py

Lines 13 to 16 in 6e2a0e2

	def test_proxy_page(helm_config, federation_url):
	r = requests.get(federation_url)
	r.raise_for_status()
	assert "How it works" in r.text

requires re-design.

The "simplest" solution is to replace assert to check for

<div id="root"></div>

from https://github.com/jupyterhub/binderhub/blob/65f807c12c61aed2179293d1bd0f72d117e92793/binderhub/templates/page.html#L24. In this simplest solution will don't need to run the JavaScript. We only check that the React based app has a HTML anchor.

Alternatively, the assert can be replaced with a solution powered by an automation library for browser testing like

• Playwright
• Selenium
• Puppeteer
• Cypress

[rgaiacs]

In #3189 (comment), @minrk suggested to use Playwright.

The continuous deployment run the test at

mybinder.org-deploy/.github/workflows/cd.yml

Lines 182 to 188 in c46ce00

	# Action Repo: https://github.com/nick-invision/retry
	- name: "Stage 4: Verify staging works"
	uses: nick-invision/retry@7152eba30c6575329ac0576536151aca5a72780e
	with:
	timeout_minutes: 10
	max_attempts: 3
	command: pytest -vx --color=yes --numprocesses=2 --release=${{ matrix.federation_member }} tests/

A couple of more steps documented at https://playwright.dev/python/docs/ci#github-actions will need to be added before the tests start.