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/db_test.go

@@ -87,9 +87,8 @@ func TestNestedInTx(t *testing.T) {
 func testSQLDB(t testing.TB) *sql.DB {
 	t.Helper()
 
-	connection, closeFn, err := dbtestutil.Open()
+	connection, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	t.Cleanup(closeFn)
 
 	db, err := sql.Open("postgres", connection)
 	require.NoError(t, err)