Feature request: Allow `toHaveStyleRule` to be called on global style

[marnixhoh]

It would be great if global style as created by createGlobalStyle, could be tested in Jest using toHaveStyleRule.

For example:

let wrapper
beforeEach(() => {
    wrapper = mount(<GlobalStyle />)
})

test('Should correctly render default GlobalStyle element', () => {
    expect(wrapper).toHaveStyleRule('background-color', 'red')
    expect(wrapper).toMatchSnapshot()
})

This has been mentioned multiple times in other issues. See #324