(core) limit retries of uploads to external store in tests

Summary:
If an external store fails completely, Grist will continue to
retry uploading to it. This diff updated the HostedStorageManager
test to limit the extent of these retries to the test itself -
otherwise they continue for all other tests in the same process,
potentially disrupting those that read logs. There are other tests
that use s3, but they aren't run in the same process with delicate
log-reading tests, and it isn't quite as clear what improvement
to make there.

Test Plan:
artificially made external store fail, and checked that
test contamination seen previously no longer occurs.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3469

app/server/lib/HostedStorageManager.ts

@@ -438,6 +438,13 @@ export class HostedStorageManager implements IDocStorageManager {
     this._uploads.expediteOperations();
   }
 
+  // forcibly stop operations that might otherwise retry indefinitely,
+  // for testing purposes.
+  public async testStopOperations() {
+    this._uploads.stopOperations();
+    await this._uploads.wait();
+  }
+
   /**
    * Finalize any operations involving the named document.
    */