Add TODO re: FailOnTimeout.builder() & JUnit 4.12

spring-test/src/main/java/org/springframework/test/context/junit4/SpringJUnit4ClassRunner.java

@@ -353,6 +353,7 @@ public class SpringJUnit4ClassRunner extends BlockJUnit4ClassRunner {
 			statement = new SpringFailOnTimeout(next, springTimeout);
 		}
 		else if (junitTimeout > 0) {
+			// TODO Use FailOnTimeout.builder() once JUnit 4.12 is the minimum supported version.
 			statement = new FailOnTimeout(next, junitTimeout);
 		}
 		else {