touchpad: hook up to the tapping configuration

Now that we have run-time changes of the tap.enabled state move the check
to the IDLE state only. Otherwise the tap machine may hang if tapping is
disabled while a gesture is in progress.

Two basic tests are added to check for the tap default setting - which is now
"tap disabled by default", for two reasons:
* if you don't know that tapping is a thing (or enabled by default), you get
  spurious button events that make the desktop feel buggy.
* if you do know what tapping is and you want it, you usually know where to
  enable it, or at least you can search for it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>