[wptrunner] Should browsers always be force-stopped?

[jonathan-j-lee]

Currently, each TestRunnerManager thread essentially uses this code to stop the browser:

def stop_runner(self, force=False):
    self.browser.stop(force=force)
    ...
    self.browser.stop(force=True)

Most browsers implement stop(force=True) as following up SIGTERM with SIGKILL if the browser doesn't shut down gracefully. #19188 added the stop(force=True) call, which seems to make the first one entirely redundant?

I think the motivation of #19188 still stands (i.e., never leak processes), so maybe wptrunner should stop supporting stop(force=False) (also, clean up the testrunner plumbing).

CC @jgraham @gsnedders