Make MockitoTestExecutionListener public

Closes gh-7016

Author: snicoll

spring-boot-test/src/main/java/org/springframework/boot/test/mock/mockito/MockitoTestExecutionListener.java

@@ -37,8 +37,9 @@
  * annotations.
  *
  * @author Phillip Webb
+ * @since 1.4.2
  */
-class MockitoTestExecutionListener extends AbstractTestExecutionListener {
+public class MockitoTestExecutionListener extends AbstractTestExecutionListener {
 
 	@Override
 	public void prepareTestInstance(TestContext testContext) throws Exception {