Tests: fix fetch mock typings for type-aware checks

src/test-utils/fetch-mock.ts

@@ -1,6 +1,13 @@
-export const withFetchPreconnect = <T extends (...args: unknown[]) => unknown>(
+type FetchWithPreconnect = {
+  preconnect: (url: string, init?: { credentials?: RequestCredentials }) => void;
+};
+
+export function withFetchPreconnect<T extends typeof fetch>(fn: T): T & FetchWithPreconnect;
+export function withFetchPreconnect<T extends object>(
   fn: T,
-): typeof fetch =>
-  Object.assign(fn, {
-    preconnect: () => {},
-  }) as unknown as typeof fetch;
+): T & FetchWithPreconnect & typeof fetch;
+export function withFetchPreconnect(fn: object) {
+  return Object.assign(fn, {
+    preconnect: (_url: string, _init?: { credentials?: RequestCredentials }) => {},
+  });
+}