message: plugins/graphiql: fix page test
diff: import React from 'react';
  import { render } from '@testing-library/react';
  import mockFetch from 'jest-fetch-mock';
  -import ExampleComponent from './ExampleComponent';
  +import { GraphiQLPage } from './GraphiQLPage';
  import { ThemeProvider } from '@material-ui/core';
  import { lightTheme } from '@backstage/theme';
  -describe('ExampleComponent', () => {
  +jest.mock('graphiql', () => () => '<GraphiQL />');
  +
  +describe('GraphiQLPage', () => {
  it('should render', () => {
  mockFetch.mockResponse(() => new Promise(() => {}));
  const rendered = render(
  <ThemeProvider theme={lightTheme}>
  - <ExampleComponent />
  + <GraphiQLPage />
  </ThemeProvider>,
  );
  - expect(rendered.getByText('Welcome to graphiql!')).toBeInTheDocument();
  + expect(rendered.getByText('GraphiQL')).toBeInTheDocument();
  + expect(rendered.getByText('<GraphiQL />')).toBeInTheDocument();
  });
  });
  