Fix bugs in handling clientRedirectUrl, and improve OIDC tests (#9127, #9128)

* 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

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/server.py b/tests/server.py

index 5a1b66270f..5a85d5fe7f 100644
--- a/tests/server.py
+++ b/tests/server.py
@@ -74,7 +74,7 @@ class FakeChannel:
         return int(self.result["code"])
 
     @property
-    def headers(self):
+    def headers(self) -> Headers:
         if not self.result:
             raise Exception("No result yet.")
         h = Headers()