// Component.js
const Component = props => (
  <MyContext.Consumer>
    {(context) => (
      <Foo
        bar={props.bar}
        baz={context.baz}
      />
    )}
  </MyContext.Consumer>
);

// Component.test.js
const outer = shallow(<Component bar="bar" />);
const Children = outer.props().children;
const wrapper = shallow(<Children baz="baz" />);

expect(wrapper.find(Foo)).toHaveLength(1);
expect(wrapper.find(Foo)).props().bar).toBe('bar');
expect(wrapper.find(Foo)).props().baz).toBe('baz');