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/handlers/target_download_test.go

@@ -24,7 +24,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
-	"sneak.berlin/go/webhooker/internal/middleware"
+	"sneak.berlin/go/webhooker/internal/middleware/middlewaretest"
 )
 
 // errClientGone is the write failure of a client that has gone away.
@@ -310,7 +310,8 @@ func limitedServer(
 	const sendBuffer = 4 << 10
 
 	logBuf := new(bytes.Buffer)
-	mw := middleware.NewForTest(
+	mw := middlewaretest.New(
+		t,
 		slog.New(slog.NewJSONHandler(logBuf, nil)),
 		&config.Config{Environment: config.EnvironmentDev},
 		nil,