* Factor out a common TestHtmlParser
Looks like I'm doing this in a few different places.
* Improve OIDC login test
Complete the OIDC login flow, rather than giving up halfway through.
* Ensure that OIDC login works with multiple OIDC providers
* Fix bugs in handling clientRedirectUrl
- don't drop duplicate query-params, or params with no value
- allow utf-8 in query-params
