Add ``--time-limit=s`` to fail long-running tests

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