(core) disentangle some server tests, release to core, add GRIST_PROXY_AUTH_HEADER test

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

app/server/declarations/tmp.d.ts

@@ -0,0 +1,8 @@
+import {Options, SimpleOptions} from "tmp";
+
+// Add declarations of the promisifies methods of tmp.
+declare module "tmp" {
+  function dirAsync(config?: Options): Promise<string>;
+  function fileAsync(config?: Options): Promise<string>;
+  function tmpNameAsync(config?: SimpleOptions): Promise<string>;
+}