Make BootstrapUtils public

See gh-28891

Author: sbrannen

spring-test/src/main/java/org/springframework/test/context/BootstrapUtils.java

@@ -44,7 +44,7 @@
  * @see BootstrapContext
  * @see TestContextBootstrapper
  */
-abstract class BootstrapUtils {
+public abstract class BootstrapUtils {
 
 	private static final String DEFAULT_BOOTSTRAP_CONTEXT_CLASS_NAME =
 			"org.springframework.test.context.support.DefaultBootstrapContext";