Make "yes" default answer on instruction

This patch changes testcase.MinimalTestcase.instruct's behaviour to the
following:

  * Input is lower cased, meaning there is no difference between "Y",
    "y", "N", or "n".

  * Default answer is considered to be yes, e.g. pressing the Enter key.

It also reduces some code duplication by reusing the same raw_input()
call since it's already in a while loop with the `response` variable
defined in the outer variable scope.