fix: make TestFn generic

Apply suggestions from code review by @MoLow

Co-authored-by: Moshe Atlow <[email protected]>

Author: hongaar

lib/test.d.ts

@@ -35,7 +35,7 @@ interface TestOptions {
   signal?: AbortSignal;
 }
 
-type TestFn = (t: TestContext, done: (result?: any) => void) => any;
+type TestFn = <T>(t: TestContext, done: (result?: T) => void) => any | <T>(t: TestContext) => Promise<T>;
 
 export default test;
 
@@ -47,13 +47,13 @@ export default test;
  *  Default: A no-op function.
  * @returns A {@link Promise} resolved with `undefined` once the test completes.
  */
-export function test(
+export function test<T>(
   name: string,
   options: TestOptions,
-  fn: TestFn
+  fn: TestFn<T>
 ): Promise<void>;
-export function test(name: string, fn: TestFn): Promise<void>;
-export function test(options: TestOptions, fn: TestFn): Promise<void>;
+export function test<T>(name: string, fn: TestFn<T>): Promise<void>;
+export function test<T>(options: TestOptions, fn: TestFn<T>): Promise<void>;
 export function test(fn: TestFn): Promise<void>;
 
 type SuiteFn = (t: SuiteContext) => void;