Add test for play button

[vulongphan]

I noticed that the play button in Toolbar.jsx is not tested in Toolbar.test.jsx and I would like to write a test for it.
@catarak Please give me some feedback on my subsequent commits. Thank you!

[vulongphan]

There seems to be two PlayIcon components in Toolbar (as attached). My understanding is that the first one does the setup and the second one starts the sketch and that when a user clicks on the play button, these two PlayIcon components are both "clicked" on. Please let me know if I have missed something.
Thank you!