[backend] Fix issues where elements sometimes would not be found with Capybara

This was *only* failing when running locally. Slightly annoying.

Turns out Capybara does not support nth-child(<num>), so we have to work around that. Work around is easy enough. Turns out then that the selenium webdriver driver does not support that work around. Hmpfh. So now we have both syntaxes used interchangeably depending on the example metadata stuff.