Adding a cq_test directive

Say I write the following in the docs:
```rst
The ``tag`` method is unusual for a Workplane method in that it returns the same Workplane object, not a new one.
```

Immediately after that I would like to write a test like
```python
a = cq.Workplane()
b = a.tag("_")
assert a == b
```

Now if that test ever fails, I can fix my statement in the docs that I have long since forgot because I have the attention span of a caffeinated chipmunk.

If we created a directive in `docs/cq_directive` called `cq_test` that instructed Sphinx to just do nothing with the content (ie. no output in docs), then we could collect them during testing like we currently do for the `cadquery` directive here: https://github.com/CadQuery/cadquery/blob/3cb72378b895398b4b5f6129f46b8866c2618067/tests/test_examples.py#L39
by adding the line:
```python
directives.register_directive("cq_test", dummy_cq_directive)
```

and now my RST would be:
```rst
The ``tag`` method is unusual for a Workplane method in that it returns the same Workplane object, not a new one.

.. cq_test::

a = cq.Workplane()
b = a.tag("_")
assert a == b
```
and pytest would run that test.

Does all that sound good?