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.

coderd/database/pubsub/pubsub_test.go

@@ -24,9 +24,8 @@ func TestPGPubsub_Metrics(t *testing.T) {
 	}
 
 	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
-	connectionURL, closePg, err := dbtestutil.Open()
+	connectionURL, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	defer closePg()
 	db, err := sql.Open("postgres", connectionURL)
 	require.NoError(t, err)
 	defer db.Close()
@@ -132,9 +131,8 @@ func TestPGPubsubDriver(t *testing.T) {
 		IgnoreErrors: true,
 	}).Leveled(slog.LevelDebug)
 
-	connectionURL, closePg, err := dbtestutil.Open()
+	connectionURL, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	defer closePg()
 
 	// use a separate subber and pubber so we can keep track of listener connections
 	db, err := sql.Open("postgres", connectionURL)