Mocks: fix button content missing

[alpha0010]

Description

Lacking button content in the provided mocks makes it impossible to write tests such as "is the button with 'foo' rendered" or "press the button with 'bar'".

Test plan

import {fireEvent, render} from '@testing-library/react-native';
import {Text} from 'react-native';
import {RectButton} from 'react-native-gesture-handler';

test('Trigger press by text', () => {
  const onPress = jest.fn();
  const {getByText} = render(
    <RectButton onPress={onPress}>
      <Text>Press Me</Text>
    </RectButton>,
  );

  fireEvent.press(getByText('Press Me'));

  expect(onPress).toHaveBeenCalled();
});

[m-bert]

Thank you for this PR ❤️

[m-bert]

Thank you!