Misc imports and tests refactoring

Python 3.6.x apparently does not properly implement PEP-0221, so `import
theano.tensor as tt` does not behave the same as `from theano.tensor as tt` and
causes the former to fail for circular imports.

This problem does not exist in Python >= 3.7.

The relevant Python issues are
- https://bugs.python.org/issue23203
- https://bugs.python.org/issue30024

Indirect references to theano.tensor imports at module level were also converted
to direct references in quite a few cases.

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