refactor(core): properly type withBody and withHead testing helpe…

…rs (#54801)

This commit addresses a typing mismatch, where these functions were declared to return whichever
value their callback returned, but this was inaccurate: it's always a test callback function
with `done` argument.

PR Close #54801

packages/private/testing/src/utils.ts

@@ -34,7 +34,10 @@ import {ɵresetJitOptions as resetJitOptions} from '@angular/core';
  * @param html HTML which should be inserted into the `body` of the `document`.
  * @param blockFn function to wrap. The function can return promise or be `async`.
  */
-export function withBody<T extends Function>(html: string, blockFn: T): T {
+export function withBody(
+  html: string,
+  blockFn: () => Promise<unknown> | void,
+): jasmine.ImplementationCallback {
   return wrapTestFn(() => document.body, html, blockFn);
 }
 
@@ -63,23 +66,26 @@ export function withBody<T extends Function>(html: string, blockFn: T): T {
  * @param html HTML which should be inserted into the `head` of the `document`.
  * @param blockFn function to wrap. The function can return promise or be `async`.
  */
-export function withHead<T extends Function>(html: string, blockFn: T): T {
+export function withHead(
+  html: string,
+  blockFn: () => Promise<unknown> | void,
+): jasmine.ImplementationCallback {
   return wrapTestFn(() => document.head, html, blockFn);
 }
 
 /**
  * Wraps provided function (which typically contains the code of a test) into a new function that
  * performs the necessary setup of the environment.
  */
-function wrapTestFn<T extends Function>(
+function wrapTestFn(
   elementGetter: () => HTMLElement,
   html: string,
-  blockFn: T,
-): T {
-  return function () {
+  blockFn: () => Promise<unknown> | void,
+): jasmine.ImplementationCallback {
+  return () => {
     elementGetter().innerHTML = html;
     return blockFn();
-  } as any;
+  };
 }
 
 /**