Let Testable annotation target any element type

Prior to this commit the usage of `@Testable` was restricted to
declarations of types, methods, and fields. As custom test engines
may use different element types as their "testable constructs",
this restriction is lifted in backward and future-compatible manner.
By dropping the `@Target` annotation, the default behaviour as
defined by the Java Language Specification applies:

  If an annotation of type java.lang.annotation.Target is not present
  on the declaration of an annotation type T, then T is applicable in
  all declaration contexts except type parameter declarations, and in
  no type contexts.

https://docs.oracle.com/javase/specs/jls/se8/html/jls-9.html#jls-9.6.4.1

Closes #2391

documentation/src/docs/asciidoc/release-notes/release-notes-5.7.0.adoc

@@ -23,7 +23,8 @@ on GitHub.
 
 ==== New Features and Improvements
 
-* ❓
+* The `@Testable` annotation may now target any element type, including fields, methods,
+  classes, packages, and modules.
 
 
 [[release-notes-5.7.0-junit-jupiter]]

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/TestableAnnotationTests.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2015-2020 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.engine;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import org.junit.jupiter.api.Test;
+import org.junit.platform.commons.annotation.Testable;
+
+/**
+ * Integration test that verifies that the testable annotation may be
+ * attached to any element type.
+ *
+ * @since 5.7
+ */
+class TestableAnnotationTests {
+
+	@Test
+	void testAndRepeatedTest() {
+		assertNotNull(new TestableEverywhere().toString());
+	}
+
+	@Testable
+	static class TestableEverywhere {
+
+		@Testable
+		final int field = 0;
+
+		@Testable
+		TestableEverywhere() {
+		}
+
+		@Testable
+		void test(@Testable int parameter) {
+			@Testable
+			var var = "var";
+		}
+	}
+
+}

junit-platform-commons/src/main/java/org/junit/platform/commons/annotation/Testable.java

@@ -13,11 +13,9 @@
 import static org.apiguardian.api.API.Status.STABLE;
 
 import java.lang.annotation.Documented;
-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;
 
 import org.apiguardian.api.API;
 
@@ -68,9 +66,13 @@
  * is therefore required to discover tests based on information specific to
  * that test engine (e.g., annotations specific to that test engine).
  *
+ * <h3>{@code @Testable} may target any declaration element type</h3>
+ * <p>Since JUnit Platform version 1.7, {@code @Testable} may target any
+ * declaration {@linkplain java.lang.annotation.ElementType element type}.
+ * That includes the aforementioned "method, field, or class" elements.
+ *
  * @since 1.0
  */
-@Target({ ElementType.TYPE, ElementType.METHOD, ElementType.FIELD })
 @Retention(RetentionPolicy.RUNTIME)
 @Inherited
 @Documented