Testing framework

[shawnpyle]

It would be great to have a framework that we could test created functions against expected results.

For example, testing

SELECT getdate();

would ensure that the added function would return a TIMESTAMP and its value is what we expected. These tests would be more complex when testing lots of inputs (see this PR).

I don't know if there is a framework to do this easily but I suspect it will require an additional dependency and the value of ensuring the functions work across postgres versions would be worth it.