add real tests

[ClaytonJY]

It would be nice to have some real checks that what we're doing is working as expected, beyond the smoke test described in #4.

We could write one or more SQL test files and have CI run them against the built container for us. We could also incorporate pgTAP but that might be overkill.

Some things to test include:

• does tsdbadmin have the right permissions? Does that remain true when various objects are created?
• can all the whitelisted extensions be enabled by a normal user, and work as expected?

This issue should be closed whenever any such tests are added; we can always add more later.