Fix FontFileUploader.test.js for using useHistory() from react-router

[masakudamatsu]

We can mock the history.push() function as follows (source: https://stackoverflow.com/a/59451956/11847654)

const mockHistoryPush = jest.fn();

jest.mock('react-router', () => ({
  ...jest.requireActual('react-router'),
  useHistory: () => ({
    push: mockHistoryPush,
  }),
}));