Add missing @Test in RestarterTests

Closes gh-13792

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/RestarterTests.java

 /*
- * Copyright 2012-2016 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.
@@ -109,6 +109,7 @@ public class RestarterTests {
 		assertThat(attribute).isEqualTo("abc");
 	}
 
+	@Test
 	public void addUrlsMustNotBeNull() throws Exception {
 		this.thrown.expect(IllegalArgumentException.class);
 		this.thrown.expectMessage("Urls must not be null");