fixes to %sqlcmd test

[edublancas]

the %sqlcmd test needs improvements in terms of docs and user experience

notebook for context: sql-test.ipynb.zip

[edublancas]

notes: we're missing the changelog entry and also the notice in the docs that states in which version this was added

[edublancas]

and this feature deserves its own tutorial, instead of embedding if in the listing tables/columns one

[gtduncan]

Just to clarify, the contents of that .ipynb should be put/paraphrased in the standalone %sqlcmd tutorial right?

[edublancas]

so there are two things here: one is fixing the logic in %sqlcmd test according to the comments in the attached notebook.

Once that's done, we need to write a tutorial showing how to use it.