Add testing models for the macros

[alittlesliceoftom]

I can open a PR for this with samples from dbt-utils.

Options:

1. We provide project that installs dbt-utils and tsql-utils in correct way, then runs the tests from dbt-utils. This may be implemented by the integration-tests code I can see floating around, but this seems to mostly be config?
2. We copy appropriate tests into this repo.

[dataders]

you've read my mind: see #4