Support detection and with test initializers

Relax `SpringBootTestContextBootstrapper` rules so that a test can
specify an `ApplicationContextInitializer` and still have
`@SpringBootConfiguration` detected.

Prior to this commit detection would not occur because it's possible
that an initializer _could_ register configuration. This scenario is
actually quite unlikely to occur, certainly less likely than wanting to
use an initializer in combination with auto-detection.

Fixes gh-8483

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/validation/DelegatingValidator.java

@@ -71,6 +71,10 @@ public class DelegatingValidator implements SmartValidator {
 		}
 	}
 
+	/**
+	 * Return the delegate validator.
+	 * @return the delegate validator
+	 */
 	protected final Validator getDelegate() {
 		return this.delegate;
 	}