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/metrics_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/middleware/middlewaretest"
 )
 
 const (
@@ -70,8 +71,8 @@ func metricsTestRouter(
 		Environment:       "prod",
 		ReceiverRateLimit: receiverLimit,
 	}
-	m := middleware.NewForTest(
-		log, cfg, newTestSessionManager(cfg, log, nil),
+	m := middlewaretest.New(
+		t, log, cfg, newTestSessionManager(t, cfg),
 	)
 
 	reg := prometheus.NewRegistry()
@@ -455,11 +456,11 @@ func TestMetrics_StatusAndSizeStillRecorded(t *testing.T) {
 	)
 }
 
-// TestMetrics_WorksOnNewForTestMiddleware pins that a Middleware built
-// by NewForTest has a recorder of its own: its Metrics() serves a
-// request instead of panicking, and a second one does not collide
-// with the first.
-func TestMetrics_WorksOnNewForTestMiddleware(t *testing.T) {
+// TestMetrics_WorksOnMiddlewaretestNew pins that a Middleware built
+// by middlewaretest.New has a recorder of its own: its Metrics()
+// serves a request instead of panicking, and a second one does not
+// collide with the first.
+func TestMetrics_WorksOnMiddlewaretestNew(t *testing.T) {
 	t.Parallel()
 
 	log := slog.New(slog.DiscardHandler)
@@ -469,7 +470,7 @@ func TestMetrics_WorksOnNewForTestMiddleware(t *testing.T) {
 	})
 
 	for range 2 {
-		h := middleware.NewForTest(log, cfg, nil).Metrics()(ok)
+		h := middlewaretest.New(t, log, cfg, nil).Metrics()(ok)
 
 		req := httptest.NewRequestWithContext(
 			t.Context(), http.MethodGet, okRoute, nil,