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.