SPI: add test extension

[urso]

Depends on #37, #38, #39

Fix import path in SPI module.

Add SPI testing extension to test suite to ensure SPI functioning correctly.

[tsg]

This is great, I'll follow up with README updates to add it to the table and such. I think we can also call the SPI support complete now in the status?

[urso]

I think we can also call the SPI support complete now in the status?

I still have some improvements for SPI support in mind. But these imrovements could be placed on top of the current APIs.

There are still a few things in SPI that we do not cover yet (or have example code for). E.g. cursor support, prepared statements, working with tuples. Maybe in we split SPI support in categories in the readme to highlight what is supported and what not.