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/database/databasetest/databasetest.go

@@ -0,0 +1,55 @@
+// Package databasetest builds a WebhookDBManager for tests in other
+// packages.
+package databasetest
+
+import (
+	"log/slog"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// NewTestWebhookDBManager creates a WebhookDBManager backed by the given
+// data directory, logging at DEBUG to standard error.
+func NewTestWebhookDBManager(
+	t *testing.T, dataDir string,
+) *database.WebhookDBManager {
+	t.Helper()
+
+	return NewTestWebhookDBManagerWithLogger(
+		t,
+		dataDir,
+		slog.New(slog.NewTextHandler(
+			os.Stderr,
+			&slog.HandlerOptions{Level: slog.LevelDebug},
+		)),
+	)
+}
+
+// NewTestWebhookDBManagerWithLogger is NewTestWebhookDBManager with the
+// logger supplied by the caller. The per-webhook databases this manager
+// opens hand that logger to gormlog, so a test that needs to see the SQL
+// the service emits can capture it.
+//
+// It is built through database.NewWebhookDBManager on a lifecycle that
+// is never started, so nothing closes its databases but the caller.
+func NewTestWebhookDBManagerWithLogger(
+	t *testing.T, dataDir string, log *slog.Logger,
+) *database.WebhookDBManager {
+	t.Helper()
+
+	mgr, err := database.NewWebhookDBManager(
+		fxtest.NewLifecycle(t),
+		database.WebhookDBManagerParams{
+			Config: &config.Config{DataDir: dataDir},
+			Logger: log,
+		},
+	)
+	require.NoError(t, err)
+
+	return mgr
+}