Make @ValidateWith only applicable to annotation types (#1612)

Add `@Target(ANNOTATION_TYPE)` to `@ValidateWith` since it's
only designed to be applied to another annotation.

Fixes #1611.

src/main/java/org/junit/validator/ValidateWith.java

@@ -1,8 +1,10 @@
 package org.junit.validator;
 
+import java.lang.annotation.ElementType;
 import java.lang.annotation.Inherited;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
 /**
  * Allows for an {@link AnnotationValidator} to be attached to an annotation.
@@ -13,6 +15,7 @@
  * @since 4.12
  */
 @Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.ANNOTATION_TYPE)
 @Inherited
 public @interface ValidateWith {
     Class<? extends AnnotationValidator> value();

src/test/java/org/junit/validator/AnnotationValidatorFactoryTest.java

@@ -14,29 +14,29 @@ public class AnnotationValidatorFactoryTest {
 
     @Test
     public void createAnnotationValidator() {
-        ValidateWith validateWith = SampleTestWithValidator.class.getAnnotation(ValidateWith.class);
+        ValidateWith validateWith = SampleAnnotationWithValidator.class.getAnnotation(ValidateWith.class);
         AnnotationValidator annotationValidator = new AnnotationValidatorFactory().createAnnotationValidator(validateWith);
         assertThat(annotationValidator, is(instanceOf(Validator.class)));
     }
 
     @ValidateWith(value = Validator.class)
-    public static class SampleTestWithValidator {
+    public @interface SampleAnnotationWithValidator {
     }
 
     public static class Validator extends AnnotationValidator {
     }
 
     @Test
     public void exceptionWhenAnnotationValidatorCantBeCreated() {
-        ValidateWith validateWith = SampleTestWithValidatorThatThrowsException.class.getAnnotation(ValidateWith.class);
+        ValidateWith validateWith = SampleAnnotationWithValidatorThatThrowsException.class.getAnnotation(ValidateWith.class);
         exception.expect(RuntimeException.class);
         exception.expectMessage("Exception received when creating AnnotationValidator class " +
                 "org.junit.validator.AnnotationValidatorFactoryTest$ValidatorThatThrowsException");
         new AnnotationValidatorFactory().createAnnotationValidator(validateWith);
     }
 
     @ValidateWith(value = ValidatorThatThrowsException.class)
-    public static class SampleTestWithValidatorThatThrowsException {
+    public @interface SampleAnnotationWithValidatorThatThrowsException {
     }
 
     public static class ValidatorThatThrowsException extends AnnotationValidator {