fix TS errors when checking unit tests

(it was loading the setup files, then following the imports into "built" directory. as those are compiled JS, it produced lots of type errors.)

packages/backend/test/jest.setup.unit.mjs

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: hazelnoot and other Sharkey contributors
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+// Need to reference the built file
+import { prepEnv } from '../built/boot/prepEnv.js';
+
+/**
+ * Applies the Sharkey environment details into the test environment.
+ * https://jestjs.io/docs/configuration#globalsetup-string
+ */
+export default function setup() {
+	// Make sure tests run in the Sharkey environment.
+	prepEnv();
+}