chore(test/v3): move bdd steps into shared module

As most of the BDD steps are shared across the V1 and V2 suites, all of
the steps have been migrated into a shared module. Due to the way
PyTest-BDD inserts and resolves these steps, the general pattern of:

```python
def outer(stacklevel: int=1):
    @when/given/then(..., stacklevel=stacklevel + 1)
    def _():
        ...
```

is required; otherwise, the steps are not available.

This commit also includes the addition of a Markdown table parser, and a
few minor changes in preparation to the V2 suite (as this commit was
created following the implementation of the full V2 consumer suite).

Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis