Remove system property dependency from MockBeanContextCachingTests

Now this test can be run regardless of the
'spring.test.context.cache.maxSize' system property value.

See gh-38236

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/mock/mockito/MockBeanContextCachingTests.java

@@ -45,7 +45,7 @@ import static org.mockito.Mockito.mock;
  */
 class MockBeanContextCachingTests {
 
-	private final DefaultContextCache contextCache = new DefaultContextCache();
+	private final DefaultContextCache contextCache = new DefaultContextCache(2);
 
 	private final DefaultCacheAwareContextLoaderDelegate delegate = new DefaultCacheAwareContextLoaderDelegate(
 			this.contextCache);