Summary:
This shuffles some server tests to make them available in grist-core,
and adds a test for the `GRIST_PROXY_AUTH_HEADER` feature added in
https://github.com/gristlabs/grist-core/pull/165
It includes a fix for a header normalization issue for websocket connections.
Test Plan: added test
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3326
