Remerged 3446d3f07946 and 0fd23055db15: the merge in 491d6aa69509 was incorrect due to badly configured merge tool

* compute_test_value=True enables on-the-fly execution of perform method
* the user provides default test values through the tag scratchpad, i.e.
T.dmatrix().tag.test_value = numpy.random.rand(4,6)

Also remove the test that used them.

We use the ShapeFeature and SpecifyShape, now,
tag.shape is being deprecated.

Thanks Fred for the review.

This avoids adding extra keys into the cache whenever these options change.