RSpec testing wants to be robust to capybara flakiness

[yannickwurm]

page.execute_script basically is too impatient and can run before the page finished loading. Credit to @tadast

So throughout the rspec, we probably need changes similar to this: https://github.com/wurmlab/sequenceserver/commit/1ed1e6354905fe23e1d70ba0bfda0f7914b1fda5#diff-d34fa42708af8d36824add[…]60f5ce14196fdaa9be6771baL144