Groovy loader should delegate to XML loader in the TCF

If a resource location in the MergedContextConfiguration has a ".xml"
extension, the GenericGroovyXmlContextLoader now delegates to a
dedicated XmlBeanDefinitionReader for loading bean definitions from that
resource, thus preserving XML validation for all XML resource locations.
For all other extensions (presumably only ".groovy"), the
GenericGroovyXmlContextLoader delegates to a GroovyBeanDefinitionReader.

Issue: SPR-11233