touchpad_test shouldn't change scroll method unless specified

The touchpad test has code to force horizontal scrolling and to change the scrolling method to edge scrolling. This makes it useless for testing e.g. two-finger scrolling. See the run() and quit() methods in the script.

Instead, it should only enable edge-scrolling if a specific switch (i.e. --edge-scrolling) is passed, otherwise it should leave the scroll-method setting alone.

None of the tests we have explicitly says it tests edge scrolling; instead, our tests generically verify that the touchpad is able to scroll in any way.