Create StagedConfigUpdateValidationTest for StagedConfigUpdate config entity type

Overview

Follow-up to 📌 Allow CMS Author to set site's homepage from navigation Postponed asked for by @wim leers. I assume since he wanted this to be "Critical" it should also be a stable blocker

Proposed resolution

Create a validation test that extends BetterConfigEntityValidationTestBase for StagedConfigUpdate config entity type. Like AssetLibraryValidationTest for the AssetLibrary entity type

User interface changes

None