Terminate a browser immediately when all tests are done on it

Currently, JTR only terminates browsers when all tests are done in all requested browsers.

It's not a problem for our CI as we only run JTR on a single browser at a time, but it makes local test runs much more annoying.

For example, running the following in jQuery:

npm run test:unit -- -v -c jtr-isolate.yml --browserstack 'IE_11' --browserstack 'Chrome_latest' --browserstack 'Firefox_latest' --browserstack 'Safari_latest'

usually leads to modern browsers finishing quickly and waiting while IE is running tests. Since they wait for a long time, the browsers are idle for too long and... JTR (or something under the hood) thinks it's because the browser got stuck, restarts the browser and re-runs the last module tests on it (here: tween).

One possible solution to this would be to fix this detection, but needlessly keeping the browser running in idle mode is also not great; just terminating the browser when it's done would solve both issues at the same time.

Start by running the npm test command shown with multiple browsers and inspect the test-runner lifecycle around completion and browser termination. Trace how Selenium and BrowserStack sessions are managed; done means each browser is terminated as soon as its tests finish without being considered stuck or rerunning its last module.