Keep test helpers out of the shipped binary (closes #506)

The four testing.go files in config, database, middleware and session
are gone. ClearEnvForTest moves to internal/config/configtest; the
webhook database manager helpers move to internal/database/databasetest
and the middleware's NewForTest to internal/middleware/middlewaretest,
both now built through the production constructors. Tests that wrapped
an open main database use database.Open. The session helpers move into
the session package's export_test.go; the middleware tests build their
session through session.New and age its timestamps instead of using a
fake clock.

The session, middleware and webhook database manager now take the
*slog.Logger they log through, so tests in other packages can give
them their own.

Model: opus-5-5

internal/delivery/archive_sweeper_test.go

@@ -21,6 +21,7 @@ import (
 	"gorm.io/gorm/clause"
 	_ "modernc.org/sqlite" // Pure Go SQLite driver.
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/database/databasetest"
 	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/gormlog"
 )
@@ -59,29 +60,14 @@ func setupArchiveTest(t *testing.T) *archiveEnv {
 	dataDir := t.TempDir()
 	log := archiveTestLogger()
 
-	sqlDB, err := sql.Open(
-		"sqlite",
-		fmt.Sprintf(
-			"file:%s?mode=rwc",
-			filepath.Join(dataDir, "main.db"),
-		),
-	)
+	mainDB, err := database.Open(dataDir, slog.New(slog.DiscardHandler))
 	require.NoError(t, err)
 
-	t.Cleanup(func() { _ = sqlDB.Close() })
-
-	gdb, err := gorm.Open(
-		sqlite.Dialector{Conn: sqlDB},
-		&gorm.Config{Logger: gormlog.New(slog.New(slog.DiscardHandler))},
-	)
-	require.NoError(t, err)
-
-	mainDB := database.NewTestDatabase(gdb)
-	require.NoError(t, mainDB.Migrate())
+	t.Cleanup(func() { _ = mainDB.Close() })
 
 	eng := delivery.NewTestEngineWithDB(
 		mainDB,
-		database.NewTestWebhookDBManager(dataDir),
+		databasetest.NewTestWebhookDBManager(t, dataDir),
 		log,
 		&http.Client{Timeout: 5 * time.Second},
 		1,