Replace the `testing.postgresql` dependency with our own implementation.
Context
testing.postgresql is an open-source library that brings up a temporary database and destroys it after testing. It's useful, and heavily used in our unit-tests.
Unfortunately, this library's last commit was in 2017, and it doesn't seem the source code is very robust: https://github.com/tk0miya/testing.postgresql/blob/master/src/testing/postgresql.py
Buildgrid should remove its dependency on this library for a custom implementation.
Acceptance Criteria
Remove and replace testing.postgresql
.
All unit tests should pass.