Polish access modifiers for test classes

See gh-27736

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/test/java/org/springframework/boot/maven/CustomLayersProviderTests.java

@@ -40,7 +40,7 @@ import static org.mockito.Mockito.mock;
  * @author Madhura Bhave
  * @author Scott Frederick
  */
-public class CustomLayersProviderTests {
+class CustomLayersProviderTests {
 
 	private CustomLayersProvider customLayersProvider;
 

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/test/java/org/springframework/boot/maven/DockerTests.java

@@ -31,7 +31,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
  * @author Wei Jiang
  * @author Scott Frederick
  */
-public class DockerTests {
+class DockerTests {
 
 	@Test
 	void asDockerConfigurationWithDefaults() {