Introduce createContext() factory method in AbstractGenericContextLoader

Prior to this commit it was possible to configure the
DefaultListableBeanFactory used by the GenericApplicationContext
created by AbstractGenericContextLoader, but it was not possible to
completely replace the bean factory.

This commit introduces a new createContext() factory method in
AbstractGenericContextLoader which indirectly allows subclasses to
supply a custom DefaultListableBeanFactory implementation to the
GenericApplicationContext.

Closes gh-25600

Author: Maciej Miklas

spring-test/src/main/java/org/springframework/test/context/support/AbstractGenericContextLoader.java

@@ -112,7 +112,7 @@ public final ConfigurableApplicationContext loadContext(MergedContextConfigurati
 
 		validateMergedContextConfiguration(mergedConfig);
 
-		GenericApplicationContext context = new GenericApplicationContext();
+		GenericApplicationContext context = createContext();
 
 		ApplicationContext parent = mergedConfig.getParentApplicationContext();
 		if (parent != null) {
@@ -130,6 +130,15 @@ public final ConfigurableApplicationContext loadContext(MergedContextConfigurati
 		return context;
 	}
 
+	/**
+	 * Creates instance of application context used by this {@code ContextLoader}
+	 *
+	 * @return new Instance of application context
+	 */
+	protected GenericApplicationContext createContext() {
+		return new GenericApplicationContext();
+	}
+
 	/**
 	 * Validate the supplied {@link MergedContextConfiguration} with respect to
 	 * what this context loader supports.
@@ -184,7 +193,7 @@ public final ConfigurableApplicationContext loadContext(String... locations) thr
 			logger.debug(String.format("Loading ApplicationContext for locations [%s].",
 				StringUtils.arrayToCommaDelimitedString(locations)));
 		}
-		GenericApplicationContext context = new GenericApplicationContext();
+		GenericApplicationContext context = createContext();
 		prepareContext(context);
 		customizeBeanFactory(context.getDefaultListableBeanFactory());
 		createBeanDefinitionReader(context).loadBeanDefinitions(locations);