This is an auxiliary host of the tests. The test page on server-headers.webtest.pocs.work embeds and fetches resources from here to test same-site and cross-site behavior. Configuring or running the tests from this host would measure everything from the wrong origin, so the configurator and the results page are not available here.
If you opened this page to accept its certificate, that is done now: you can close this tab and reload the test page.