feat(testing): add overrideTemplate method (#13372)

Closes #10685

modules/@angular/core/testing/test_bed.ts

@@ -135,6 +135,11 @@ export class TestBed implements Injector {
     return TestBed;
   }
 
+  static overrideTemplate(component: Type<any>, template: string): typeof TestBed {
+    getTestBed().overrideComponent(component, {set: {template, templateUrl: null}});
+    return TestBed;
+  }
+
   static get(token: any, notFoundValue: any = Injector.THROW_IF_NOT_FOUND) {
     return getTestBed().get(token, notFoundValue);
   }

modules/@angular/platform-browser/test/testing_public_spec.ts

@@ -8,7 +8,7 @@
 
 import {CompilerConfig, ResourceLoader} from '@angular/compiler';
 import {CUSTOM_ELEMENTS_SCHEMA, Component, Directive, Injectable, Input, NgModule, Pipe} from '@angular/core';
-import {TestBed, async, fakeAsync, inject, tick, withModule} from '@angular/core/testing';
+import {TestBed, async, fakeAsync, getTestBed, inject, tick, withModule} from '@angular/core/testing';
 import {expect} from '@angular/platform-browser/testing/matchers';
 
 import {stringify} from '../src/facade/lang';
@@ -356,6 +356,21 @@ export function main() {
             expect(compFixture.nativeElement).toHaveText('transformed hello');
           });
         });
+
+        describe('template', () => {
+          let testBedSpy: any;
+          beforeEach(() => {
+            testBedSpy = spyOn(getTestBed(), 'overrideComponent').and.callThrough();
+            TestBed.overrideTemplate(SomeComponent, 'newText');
+          });
+          it(`should override component's template`, () => {
+            const fixture = TestBed.createComponent(SomeComponent);
+            expect(fixture.nativeElement).toHaveText('newText');
+            expect(testBedSpy).toHaveBeenCalledWith(SomeComponent, {
+              set: {template: 'newText', templateUrl: null}
+            });
+          });
+        });
       });
 
       describe('setting up the compiler', () => {

tools/public_api_guard/core/testing/index.d.ts

@@ -89,6 +89,7 @@ export declare class TestBed implements Injector {
     static overrideDirective(directive: Type<any>, override: MetadataOverride<Directive>): typeof TestBed;
     static overrideModule(ngModule: Type<any>, override: MetadataOverride<NgModule>): typeof TestBed;
     static overridePipe(pipe: Type<any>, override: MetadataOverride<Pipe>): typeof TestBed;
+    static overrideTemplate(component: Type<any>, template: string): typeof TestBed;
     /** @experimental */ static resetTestEnvironment(): void;
     static resetTestingModule(): typeof TestBed;
 }