test(web): share one browser across the key-pad test classes

Each class launched its own chromium, and `unittest discover` runs this file
alongside test_web_history.py in a single process. Four concurrent browsers was
enough to make `page.goto` time out intermittently -- a full tests/run.sh
failed once with "Ran 0 tests, errors=5" and passed on the retry. One browser
per module now; pages stay per class, so a test that resizes the viewport or
leaves a panel open still cannot reach another class.