Documentation / Testing: Mock unit test calls to postgres, remove PG start in contributing

[KwilLuke]

Suggestion

Right now, some of the unit tests call Postgres directly; this requires that the tester is running a Postgres instance.

To close this issue, we will need to:

1. Mock the PG response in the unit tests that are currently calling PG.
2. Remove the instruction in Contributing.md that tells the tester to start PG before running the unit tests.

[jchappelow]

For certain packages we will mock a higher level type that provides say a Datastore, and in others the resolution will be to mock the actual pg response if the code in question is tightly coupled with the database backend (internal/sql/pg package) and/or pgx.

It depends what we are aiming to test in a given unit tests.