Force and keep browser open option

Add `--force` option that kills already running browser. We also need an option that keeps the browser open instead of closing it.

Start by locating the command-line option parsing and the browser startup and shutdown flow. Check how an already running browser is detected and how the browser is closed after tests; done means `--force` can terminate the existing browser and a separate option reliably leaves the browser open.

Written by the indexing model from the issue text.