Add tests for managing focus when opening/closing sidebar

[meganesu]

In app/index.spec.js, there are tests that still needs to be implemented:

• it.todo('moves focus to the sidebarRef');
• it.todo('moves focus back to the lastClickedInstructionRef');

Tried so far:

• Using jest.spyOn.mockReturnValue to replace the ref returned from useRef() with something like { current: focus: jest.fn() }. This worked for overwriting the initial value of sidebarRef, but once that ref gets set to the h3 element inside of DetailsSidebar, current gets overwritten again with the actual element.

[theashraf]

I'd like to help with this one

[meganesu]

@theashraf sounds good, I'll assign this issue to you. Thanks!