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/dbtestutil/db.go

@@ -95,21 +95,17 @@ func NewDB(t testing.TB, opts ...Option) (database.Store, pubsub.Pubsub) {
 		opt(&o)
 	}
 
-	db := dbmem.New()
-	ps := pubsub.NewInMemory()
+	var db database.Store
+	var ps pubsub.Pubsub
 	if WillUsePostgres() {
 		connectionURL := os.Getenv("CODER_PG_CONNECTION_URL")
 		if connectionURL == "" && o.url != "" {
 			connectionURL = o.url
 		}
 		if connectionURL == "" {
-			var (
-				err     error
-				closePg func()
-			)
-			connectionURL, closePg, err = Open()
+			var err error
+			connectionURL, err = Open(t)
 			require.NoError(t, err)
-			t.Cleanup(closePg)
 		}
 
 		if o.fixedTimezone == "" {
@@ -143,6 +139,9 @@ func NewDB(t testing.TB, opts ...Option) (database.Store, pubsub.Pubsub) {
 		t.Cleanup(func() {
 			_ = ps.Close()
 		})
+	} else {
+		db = dbmem.New()
+		ps = pubsub.NewInMemory()
 	}
 
 	return db, ps