Replace deprecated TestClient with fetchMock (#3550)
* replace deprecated TestClient with fetchMock
* add stop() api to BackupManager for clean shutdown
* fix merge
* code review cleaning
* lint
* Address review comments
* Remove unused `TestClient.expectKeyBackupQuery`
* clean up imports
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Co-authored-by: Richard van der Hoff <richard@matrix.org>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
