(core) move apiserver tests to core, disentangling notifier+billing parts

Summary:
This moves some more tests to core that would be useful for ANCT,
which had been stuck in grist-saas due to some entanglements with
sendgrid and billing. For sendgrid, I've moved around just enough
material to permit the tests to run mostly unchanged. Ideally
the interface to a notification system would be generalized, but
that's a bigger project.

Test Plan:
checked that tests are likely to run as expected
in core using preview laid out by ./buildtools/build_core.sh

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4149

app/server/lib/ICreate.ts

@@ -97,6 +97,7 @@ export function makeSimpleCreator(opts: {
       return notifier?.create(dbManager, gristConfig) ?? {
         get testPending() { return false; },
         async deleteUser()      { /* do nothing */ },
+        testSetSendMessageCallback() { return undefined; },
       };
     },
     ExternalStorage(purpose, extraPrefix) {

app/server/lib/INotifier.ts

@@ -1,6 +1,12 @@
+import {SendGridConfig, SendGridMail} from 'app/gen-server/lib/NotifierTypes';
+
 export interface INotifier {
   // for test purposes, check if any notifications are in progress
   readonly testPending: boolean;
 
   deleteUser(userId: number): Promise<void>;
+
+  // Intercept outgoing messages for test purposes.
+  // Return undefined if no notification system is available.
+  testSetSendMessageCallback(op: (body: SendGridMail, description: string) => Promise<void>): SendGridConfig|undefined;
 }