These `__init__.py` modules contained multiple confusing import orders and
assumed a few indirect imports (e.g. `tensor` in `theano/__init__.py` and `cc`
in `theano/gof/__init__.py`) that unnecessarily confounded static analysis
tools.

Move get_test_value method to the Variable (sub)classes

The test value computation steps were also extracted from `PureOp.__call__` and
put into a stand-alone `compute_test_value` function.

Refactor test value framework so that test value validation is performed up-front.

This test simply loaded an old example of pickled data.  It was both extremely
indirect and brittle.  Changes to the code base will eventually invalidate
nearly all of the old classes used, and whatever the exact issue was, it
should've been tested directly.

The temporary values created during subtensor broadcast-value computations would
cause the test value computation steps to complain/err.  This change prevents
that.