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/middleware/middlewaretest/middlewaretest.go

@@ -0,0 +1,42 @@
+// Package middlewaretest builds a Middleware for tests in other
+// packages.
+package middlewaretest
+
+import (
+	"log/slog"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/session"
+)
+
+// New builds a Middleware through middleware.New, on a
+// lifecycle that is never started.
+//
+// Its metrics recorder writes to a fresh registry of its own, so
+// Metrics() works on it and two of them never collide.
+func New(
+	t *testing.T,
+	log *slog.Logger,
+	cfg *config.Config,
+	sess *session.Session,
+) *middleware.Middleware {
+	t.Helper()
+
+	m, err := middleware.New(
+		fxtest.NewLifecycle(t),
+		middleware.MiddlewareParams{
+			Logger:   log,
+			Config:   cfg,
+			Session:  sess,
+			Registry: prometheus.NewRegistry(),
+		},
+	)
+	require.NoError(t, err)
+
+	return m
+}