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 as
ClearEnv; the webhook database manager helpers move to
internal/database/databasetest as NewWebhookDBManager and
NewWebhookDBManagerWithLogger, and the middleware's NewForTest to
internal/middleware/middlewaretest as New, 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/database/webhook_db_manager_test.go

@@ -18,6 +18,7 @@ import (
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/database/databasetest"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
@@ -50,7 +51,7 @@ func setupTestWebhookDBManager(
 		lc,
 		database.WebhookDBManagerParams{
 			Config: cfg,
-			Logger: l,
+			Logger: l.Get(),
 		},
 	)
 	require.NoError(t, err)
@@ -117,7 +118,8 @@ func TestWebhookDBManager_ConcurrentFirstTouchOpensOnce(t *testing.T) {
 
 	var logs bytes.Buffer
 
-	mgr := database.NewTestWebhookDBManagerWithLogger(
+	mgr := databasetest.NewWebhookDBManagerWithLogger(
+		t,
 		t.TempDir(),
 		slog.New(slog.NewTextHandler(&logs, nil)),
 	)
@@ -307,7 +309,8 @@ func TestWebhookDBManager_LostDatabaseIsLogged(t *testing.T) {
 
 		var logs bytes.Buffer
 
-		mgr := database.NewTestWebhookDBManagerWithLogger(
+		mgr := databasetest.NewWebhookDBManagerWithLogger(
+			t,
 			t.TempDir(),
 			slog.New(slog.NewTextHandler(&logs, nil)),
 		)