Add tests to xeus-sql

[marimeireles]

There are two kinds of tests I'd like to add here:

Notebook tests to xeus-sql

It's not very easy to setup an instance of working jupyter notebooks on CI -because of the many integrations, but we can do it for sqlite. Here is an example of what I'd like to have.

GTEST

We have everything setup to build and run gtests on our cmake we just need to build them. The biggest the coverage the better, feel free to add a list here or contribute with any tests! :)

I'll try to add some slowly but I'll gladly review PRs! Thanks! 🌸