Support XML config fully in web integration tests

Prior to this commit, it was impossible to use all features of XML
configuration (e.g., the <qualifier> tag) in web-based integration
tests (loaded using @WebAppConfiguration, @ContextConfiguration, etc.)
if the Groovy library was on the classpath. The reason is that the
GroovyBeanDefinitionReader used internally by
GenericGroovyXmlWebContextLoader disables XML validation for its
internal XmlBeanDefinitionReader, and this prevents some XML
configuration features from working properly. For example, the default
value for the 'type' attribute (defined in the spring-beans XSD) of the
<qualifier> tag gets ignored, resulting in an exception when the
application context is loaded.

This commit addresses this issue by refactoring the implementation of
loadBeanDefinitions() in GenericGroovyXmlWebContextLoader to use an
XmlBeanDefinitionReader or GroovyBeanDefinitionReader depending on the
file extension of the resource location from which bean definitions
should be loaded. This aligns the functionality of
GenericGroovyXmlWebContextLoader with the existing functionality of
GenericGroovyXmlContextLoader.

Issue: SPR-12768

spring-test/src/test/resources/org/springframework/test/context/web/BasicXmlWacTests-context.xml

@@ -2,6 +2,8 @@
 <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xmlns:c="http://www.springframework.org/schema/c" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
 
-	<bean id="foo" class="java.lang.String" c:_="bar" />
+	<bean id="foo" class="java.lang.String" c:_="bar">
+		<qualifier value="foo" />
+	</bean>
 
 </beans>