Introduce logging for ContextCustomizer[Factory] to improve diagnostics

Prior to this commit, we logged which default TestExecutionListeners
were discovered and which listeners were used, but we did not log
anything for ContextCustomizerFactory and ContextCustomizer.

This commit therefore introduces similar logging for
ContextCustomizerFactory and ContextCustomizer to improve diagnostics.

Closes gh-29036

Author: sbrannen

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

@@ -202,13 +202,10 @@ public final List<TestExecutionListener> getTestExecutionListeners() {
 	protected List<TestExecutionListener> getDefaultTestExecutionListeners() {
 		List<TestExecutionListener> listeners = SpringFactoriesLoader.forDefaultResourceLocation()
 				.load(TestExecutionListener.class, this::handleListenerInstantiationFailure);
-
-		if (logger.isInfoEnabled()) {
-			List<String> classNames = listeners.stream().map(Object::getClass).map(Class::getName).toList();
-			logger.info("Loaded default TestExecutionListener implementations from location [%s]: %s"
-					.formatted(SpringFactoriesLoader.FACTORIES_RESOURCE_LOCATION, classNames));
+		if (logger.isDebugEnabled()) {
+			logger.debug("Loaded default TestExecutionListener implementations from location [%s]: %s"
+					.formatted(SpringFactoriesLoader.FACTORIES_RESOURCE_LOCATION, classNames(listeners)));
 		}
-
 		return Collections.unmodifiableList(listeners);
 	}
 
@@ -426,6 +423,9 @@ private Set<ContextCustomizer> getContextCustomizers(Class<?> testClass,
 				customizers.add(customizer);
 			}
 		}
+		if (logger.isInfoEnabled()) {
+			logger.info("Using ContextCustomizers: " + customizers);
+		}
 		return customizers;
 	}
 
@@ -438,7 +438,13 @@ private Set<ContextCustomizer> getContextCustomizers(Class<?> testClass,
 	 * @see SpringFactoriesLoader#loadFactories
 	 */
 	protected List<ContextCustomizerFactory> getContextCustomizerFactories() {
-		return SpringFactoriesLoader.loadFactories(ContextCustomizerFactory.class, getClass().getClassLoader());
+		List<ContextCustomizerFactory> factories =
+				SpringFactoriesLoader.loadFactories(ContextCustomizerFactory.class, getClass().getClassLoader());
+		if (logger.isDebugEnabled()) {
+			logger.debug("Loaded ContextCustomizerFactory implementations from location [%s]: %s"
+					.formatted(SpringFactoriesLoader.FACTORIES_RESOURCE_LOCATION, classNames(factories)));
+		}
+		return factories;
 	}
 
 	/**
@@ -562,6 +568,10 @@ protected MergedContextConfiguration processMergedContextConfiguration(MergedCon
 	}
 
 
+	private static List<String> classNames(List<?> components) {
+		return components.stream().map(Object::getClass).map(Class::getName).toList();
+	}
+
 	private static boolean areAllEmpty(Collection<?>... collections) {
 		return Arrays.stream(collections).allMatch(Collection::isEmpty);
 	}