relates to https://github.com/coder/internal/issues/927
Refactors dbtestutil to use a `Broker` struct to create test databases. Additionally uses a `coder_testing` database to record test databases that are created and when they are dropped.
This is in preparation for the PR above this in the stack which adds a "cleaner" subprocess that cleans out any databases that were left when the test process ends.
Spike Curtis