config-file-provider plugin provides a set of custom implementations of Config:

• GlobalMavenSettingsConfig
• MavenSettingsConfig
• MavenToolchainsConfig
• GroovyScript
• JsonConfig
• XmlConfig
• CustomConfig

All those configurations are tested by config-file-provider. Also, others plugins (like managed-scripts) make use of Extensions Points and test their extensions. For a core feature it should be exercised by the plugin's own tests.