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.

cli/resetpassword_test.go

@@ -32,9 +32,8 @@ func TestResetPassword(t *testing.T) {
 	const newPassword = "MyNewPassword!"
 
 	// start postgres and coder server processes
-	connectionURL, closeFunc, err := dbtestutil.Open()
+	connectionURL, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	defer closeFunc()
 	ctx, cancelFunc := context.WithCancel(context.Background())
 	serverDone := make(chan struct{})
 	serverinv, cfg := clitest.New(t,

cli/server_createadminuser_test.go

@@ -85,9 +85,8 @@ func TestServerCreateAdminUser(t *testing.T) {
 			// Skip on non-Linux because it spawns a PostgreSQL instance.
 			t.SkipNow()
 		}
-		connectionURL, closeFunc, err := dbtestutil.Open()
+		connectionURL, err := dbtestutil.Open(t)
 		require.NoError(t, err)
-		defer closeFunc()
 
 		sqlDB, err := sql.Open("postgres", connectionURL)
 		require.NoError(t, err)
@@ -151,9 +150,8 @@ func TestServerCreateAdminUser(t *testing.T) {
 			// Skip on non-Linux because it spawns a PostgreSQL instance.
 			t.SkipNow()
 		}
-		connectionURL, closeFunc, err := dbtestutil.Open()
+		connectionURL, err := dbtestutil.Open(t)
 		require.NoError(t, err)
-		defer closeFunc()
 
 		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitMedium)
 		defer cancel()
@@ -185,9 +183,8 @@ func TestServerCreateAdminUser(t *testing.T) {
 			// Skip on non-Linux because it spawns a PostgreSQL instance.
 			t.SkipNow()
 		}
-		connectionURL, closeFunc, err := dbtestutil.Open()
+		connectionURL, err := dbtestutil.Open(t)
 		require.NoError(t, err)
-		defer closeFunc()
 
 		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitMedium)
 		defer cancel()
@@ -225,9 +222,8 @@ func TestServerCreateAdminUser(t *testing.T) {
 			// Skip on non-Linux because it spawns a PostgreSQL instance.
 			t.SkipNow()
 		}
-		connectionURL, closeFunc, err := dbtestutil.Open()
+		connectionURL, err := dbtestutil.Open(t)
 		require.NoError(t, err)
-		defer closeFunc()
 		ctx, cancelFunc := context.WithCancel(context.Background())
 		defer cancelFunc()
 

cli/server_test.go

@@ -1598,9 +1598,8 @@ func TestServer_Production(t *testing.T) {
 		// Skip on non-Linux because it spawns a PostgreSQL instance.
 		t.SkipNow()
 	}
-	connectionURL, closeFunc, err := dbtestutil.Open()
+	connectionURL, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	defer closeFunc()
 
 	// Postgres + race detector + CI = slow.
 	ctx, cancelFunc := context.WithTimeout(context.Background(), testutil.WaitSuperLong*3)
@@ -1803,9 +1802,8 @@ func TestConnectToPostgres(t *testing.T) {
 
 	log := slogtest.Make(t, nil)
 
-	dbURL, closeFunc, err := dbtestutil.Open()
+	dbURL, err := dbtestutil.Open(t)
 	require.NoError(t, err)
-	t.Cleanup(closeFunc)
 
 	sqlDB, err := cli.ConnectToPostgres(ctx, log, "postgres", dbURL)
 	require.NoError(t, err)