Add config to tests in various places

Signed-off-by: David Mehren <git@herrmehren.de>

src/config/app.config.mock.ts

@@ -0,0 +1,19 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { registerAs } from '@nestjs/config';
+
+export default registerAs('appConfig', () => ({
+  port: 3000,
+  media: {
+    backend: {
+      use: 'filesystem',
+      filesystem: {
+        uploadPath: 'uploads',
+      },
+    },
+  },
+}));