Unify method visibility of private classes

Apply checkstyle rule to ensure that private and package private
classes do not have unnecessary public methods. Test classes have
also been unified as much as possible to use default scoped
inner-classes.

Closes gh-7316

spring-boot-project/spring-boot-tools/spring-boot-autoconfigure-processor/src/test/java/org/springframework/boot/autoconfigureprocessor/TestClassConfiguration.java

@@ -32,7 +32,7 @@ import org.springframework.boot.autoconfigureprocessor.TestConditionalOnWebAppli
 public class TestClassConfiguration {
 
 	@TestAutoConfigureOrder
-	public static class Nested {
+	static class Nested {
 
 	}