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/gen/dump/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"database/sql"
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -12,12 +13,34 @@ import (
 
 var preamble = []byte("-- Code generated by 'make coderd/database/generate'. DO NOT EDIT.")
 
+type mockTB struct {
+	cleanup []func()
+}
+
+func (t *mockTB) Cleanup(f func()) {
+	t.cleanup = append(t.cleanup, f)
+}
+
+func (*mockTB) Helper() {
+	// noop
+}
+
+func (*mockTB) Logf(format string, args ...any) {
+	_, _ = fmt.Printf(format, args...)
+}
+
 func main() {
-	connection, closeFn, err := dbtestutil.Open()
+	t := &mockTB{}
+	defer func() {
+		for _, f := range t.cleanup {
+			f()
+		}
+	}()
+
+	connection, err := dbtestutil.Open(t)
 	if err != nil {
 		panic(err)
 	}
-	defer closeFn()
 
 	db, err := sql.Open("postgres", connection)
 	if err != nil {