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/clientip_test.go

@@ -12,6 +12,7 @@ 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 (
@@ -133,8 +134,8 @@ func clientLogLines(
 		TrustedProxies:    trustedProxies(trustedProxyCIDR),
 	}
 
-	m := middleware.NewForTest(
-		log, cfg, newTestSessionManager(cfg, log, nil),
+	m := middlewaretest.NewForTest(
+		t, log, cfg, newTestSessionManager(t, cfg),
 	)
 	handler := m.Logging()(site.build(m))