checkbox-touch: fix session not being cleared

This patch makes checkbox-touch controller clear current session data
after testing session is completed.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1382533
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge by tarmac [r=zkrynicki][bug=1382533][author=kissiel]"