chore: add postgres template caching for tests (#15336)

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.

Makefile

@@ -765,7 +765,7 @@ sqlc-vet: test-postgres-docker
 test-postgres: test-postgres-docker
 	# The postgres test is prone to failure, so we limit parallelism for
 	# more consistent execution.
-	$(GIT_FLAGS)  DB=ci DB_FROM=$(shell go run scripts/migrate-ci/main.go) gotestsum \
+	$(GIT_FLAGS)  DB=ci gotestsum \
 		--junitfile="gotests.xml" \
 		--jsonfile="gotests.json" \
 		--packages="./..." -- \