add getTextContent to MatTabHarness

Author: mmalerba

src/material/tabs/testing/shared.spec.ts

@@ -99,6 +99,7 @@ export function runHarnessTests(
   it('should be able to get harness loader for content element of active tab', async () => {
     const tabGroup = await loader.getHarness(tabGroupHarness);
     const tabs = await tabGroup.getTabs();
+    expect(await tabs[0].getTextContent()).toBe('Content 1');
     const tabContentLoader = await tabs[0].getHarnessLoaderForContent();
     const tabContentHarness = await tabContentLoader.getHarness(TestTabContentHarness);
     expect(await (await tabContentHarness.host()).text()).toBe('Content 1');

src/material/tabs/testing/tab-harness.ts

@@ -60,6 +60,13 @@ export class MatTabHarness extends ComponentHarness {
     await (await this.host()).click();
   }
 
+  /** Gets the text content of the tab. */
+  async getTextContent(): Promise<string> {
+    const contentId = await this._getContentId();
+    const contentEl = await this.documentRootLocatorFactory().locatorFor(`#${contentId}`)();
+    return contentEl.text();
+  }
+
   /**
    * Gets a `HarnessLoader` that can be used to load harnesses for components within the tab's
    * content area.