Add configuration for testable images

Depends on http://review.merproject.org/1483

Open questions:
 - I simply named it "Testable" - any suggestion of a better name?
 - Which configurations to provide in "Testable" variant?
 - Maybe "RND" should be also "Testable", i.e., inst. of "RND"+"RND Testable" having just "RND" which inherits "Mer Testing"?