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/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.NewForTest(
+		t, log, cfg, newTestSessionManager(t, cfg),
 	)
 
 	reg := prometheus.NewRegistry()
@@ -456,9 +457,9 @@ 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.
+// by middlewaretest.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) {
 	t.Parallel()
 
@@ -469,7 +470,7 @@ func TestMetrics_WorksOnNewForTestMiddleware(t *testing.T) {
 	})
 
 	for range 2 {
-		h := middleware.NewForTest(log, cfg, nil).Metrics()(ok)
+		h := middlewaretest.NewForTest(t, log, cfg, nil).Metrics()(ok)
 
 		req := httptest.NewRequestWithContext(
 			t.Context(), http.MethodGet, okRoute, nil,