This PR is the first in a series aimed at closing
[#15109](https://github.com/coder/coder/issues/15109).
### Changes
- **Template Database Creation:**
`dbtestutil.Open` now has the ability to create a template database if
none is provided via `DB_FROM`. The template database’s name is derived
from a hash of the migration files, ensuring that it can be reused
across tests and is automatically updated whenever migrations change.
- **Optimized Database Handling:**
Previously, `dbtestutil.Open` would spin up a new container for each
test when `DB_FROM` was unset. Now, it first checks for an active
PostgreSQL instance on `localhost:5432`. If none is found, it creates a
single container that remains available for subsequent tests,
eliminating repeated container startups.
These changes address the long individual test times (10+ seconds)
reported by some users, likely due to the time Docker took to start and
complete migrations.
