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/database/webhook_db_manager.go

@@ -15,7 +15,6 @@ import (
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/datadir"
 	"sneak.berlin/go/webhooker/internal/gormlog"
-	"sneak.berlin/go/webhooker/internal/logger"
 )
 
 // WebhookDBManagerParams holds the fx dependencies for
@@ -24,7 +23,7 @@ type WebhookDBManagerParams struct {
 	fx.In
 
 	Config *config.Config
-	Logger *logger.Logger
+	Logger *slog.Logger
 }
 
 // errInvalidCachedDBType indicates a type assertion failure
@@ -70,7 +69,7 @@ func NewWebhookDBManager(
 ) (*WebhookDBManager, error) {
 	m := &WebhookDBManager{
 		dataDir: params.Config.DataDir,
-		log:     params.Logger.Get(),
+		log:     params.Logger,
 	}
 
 	// Create data directory if it doesn't exist. datadir.DirPerm is the