According to the document at /core/tests/README.md, we could force all BrowserTestBase (including legacy JavascriptTestBase) tests to use
webdriver by setting the value of MINK_DRIVER_CLASS. While the driver parameters defined by MINK_DRIVER_ARGS_WEBDRIVER would not be loaded as expected. It's still the default parameters for PhantomJSDriver.
The fix is to reload the parameters after switching the driver. The patch is attached below.