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/target_database_rotation_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/database"
+	"sneak.berlin/go/webhooker/internal/database/databasetest"
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
@@ -335,7 +336,7 @@ func TestArchivePathAt(t *testing.T) {
 	t.Parallel()
 
 	dataDir := t.TempDir()
-	dbMgr := database.NewTestWebhookDBManager(dataDir)
+	dbMgr := databasetest.NewTestWebhookDBManager(t, dataDir)
 	webhook := &database.Webhook{
 		BaseModel: database.BaseModel{ID: "wh-id"}, Name: "Orders",
 	}