Only check for Mockito once

Update `MockReset` so that checking for Mockito only happens once.

Closes gh-14260

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

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,6 +53,9 @@ public enum MockReset {
 	 */
 	NONE;
 
+	private static final boolean MOCKITO_PRESENT = ClassUtils
+			.isPresent("org.mockito.internal.util.MockUtil", null);
+
 	/**
 	 * Create {@link MockSettings settings} to be used with mocks where reset should occur
 	 * before each test method runs.
@@ -103,7 +106,7 @@ public enum MockReset {
 	@SuppressWarnings("rawtypes")
 	static MockReset get(Object mock) {
 		MockReset reset = MockReset.NONE;
-		if (ClassUtils.isPresent("org.mockito.internal.util.MockUtil", null)) {
+		if (MOCKITO_PRESENT) {
 			if (Mockito.mockingDetails(mock).isMock()) {
 				MockCreationSettings settings = MockitoApi.get().getMockSettings(mock);
 				List listeners = settings.getInvocationListeners();