Define test helpers for more thorough testing

[colearendt]

Specifically, it would be nice to have some of the standard con objects available for every test, so we are not having to re-build them each time.

Further, it would be great to have some wrappers for determining if DSNs are available, if Postgres is available (where would the configuration be stored, etc.). This way, we could easily skip tests when infrastructure is not available (i.e. on CRAN), but have a more thorough testing suite when developing, as well.