Position each actuator endpoint documentation test class alongside its endpoint auto-configuration class

[wilkinsona]

The org.springframework.boot.actuate.autoconfigure.endpoint.web.documentation package currently knows about every Actuator endpoint. Rearrange things so that the package only provides the base infrastructure for documenting Actuator endpoints by moving each of the concrete endpoint-specific test classes into the same package as the auto-configuration for the endpoint that it documents.