test(@angular-devkit/schematics): add explicit return types for isolated declarations

This commit adds explicit return types to schematic factory functions in the test suite to satisfy the `isolatedDeclarations` requirement.

Author: clydin

tests/angular_devkit/schematics/tools/file-system-engine-host/extra-properties/factory.ts

@@ -10,7 +10,7 @@
 import { SchematicContext, Tree } from '@angular-devkit/schematics';
 
 export default function (options: {}) {
-  return (tree: Tree, context: SchematicContext) => {
+  return (tree: Tree, context: SchematicContext): void => {
     // We pass information back to the test.
     tree.create(
       (context.schematic.description as any).extra, // tslint:disable-line:no-any

tests/angular_devkit/schematics/tools/file-system-engine-host/file-tasks/factory.ts

@@ -10,7 +10,7 @@
 import { SchematicContext, Tree } from '@angular-devkit/schematics';
 
 export default function () {
-  return (_: Tree, context: SchematicContext) => {
+  return (_: Tree, context: SchematicContext): void => {
     context.addTask({
       toConfiguration() {
         return { name: 'file-tasks/file-task.js' };

tests/angular_devkit/schematics/tools/file-system-engine-host/null-factory.ts

@@ -6,5 +6,5 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 export default function () {
-  return () => {};
+  return (): void => {};
 }