SE-1917 Replace hacky sleeps with webdriver waits

[samuelallan72]

The tests don't appear to be flaky on my machine with python 3 any more, but this at least removes the hacky manual sleeps where possible. Instead it relies on webdriver waiting for changes to happen (element to exist, become visible, etc.). This will be faster and more reliable.

Test instructions:

• sanity check the code to make sure the test semantics haven't been changed
• verify that tests in CI reliably pass

Reviewer:

• @pomegranited

[pomegranited]

👍 LGTM @swalladge !

• I tested this by ensuring the automated tests had passed consistently.
• I read through the code changes, and checked all the places in the tests where sleep is still invoked, to ensure there were no better options available.
• I checked for accessibility issues test changes only
• Includes documentation -- adds code comments where sleep was still required.
• I made sure any change in configuration variables is reflected in the corresponding
  client's configuration-secure repository. N/A