Improve unit test stability

Author: wojtekmaj

src/DateTimePicker.spec.jsx

@@ -158,23 +158,23 @@ describe('DateTimePicker', () => {
 
     const { container } = render(<DateTimePicker className={className} />);
 
-    const wrapper = container.firstChild;
+    const wrapper = container.firstElementChild;
 
     expect(wrapper).toHaveClass(className);
   });
 
   it('applies "--open" className to its wrapper when given isCalendarOpen flag', () => {
     const { container } = render(<DateTimePicker isCalendarOpen />);
 
-    const wrapper = container.firstChild;
+    const wrapper = container.firstElementChild;
 
     expect(wrapper).toHaveClass('react-datetime-picker--open');
   });
 
   it('applies "--open" className to its wrapper when given isClockOpen flag', () => {
     const { container } = render(<DateTimePicker isClockOpen />);
 
-    const wrapper = container.firstChild;
+    const wrapper = container.firstElementChild;
 
     expect(wrapper).toHaveClass('react-datetime-picker--open');
   });