feat(core): add support config & button/error components

This adds a new app.support top level config that can be used to
configure various support links and other information. This can later be
extended for plugins to append their own specific support items.

plugins/welcome/src/components/WelcomePage/WelcomePage.test.tsx

@@ -14,23 +14,22 @@
  * limitations under the License.
  */
 
-import React from 'react';
-import { render } from '@testing-library/react';
-import WelcomePage from './WelcomePage';
-import { ThemeProvider } from '@material-ui/core';
-import { lightTheme } from '@backstage/theme';
 import {
   ApiProvider,
   ApiRegistry,
-  errorApiRef,
   configApiRef,
   ConfigReader,
+  errorApiRef,
 } from '@backstage/core';
+import { renderInTestApp } from '@backstage/test-utils';
+import { lightTheme } from '@backstage/theme';
+import { ThemeProvider } from '@material-ui/core';
+import React from 'react';
+import WelcomePage from './WelcomePage';
 
 describe('WelcomePage', () => {
-  it('should render', () => {
-    // TODO: use common test app with mock implementations of all core APIs
-    const rendered = render(
+  it('should render', async () => {
+    const { baseElement } = await renderInTestApp(
       <ApiProvider
         apis={ApiRegistry.from([
           [errorApiRef, { post: jest.fn() }],
@@ -42,6 +41,6 @@ describe('WelcomePage', () => {
         </ThemeProvider>
       </ApiProvider>,
     );
-    expect(rendered.baseElement).toBeInTheDocument();
+    expect(baseElement).toBeInTheDocument();
   });
 });