Add --time-limit=s to fail long-running tests
- This removes and replaces pytest-timeout, which wasn't thread-safe
Usage (command-line):
--time-limit=s (fail the tests that run longer than "s" seconds)
Usage (in-test):
self.set_time_limit(s) (fail that test if it runs longer than "s" seconds)
Other unrelated changes:
- Better detection of multi-thread use
- Update click_visible_elements()
- Some minor refactoring