Introduce @Suite for TestNG tests

In order to allow developers to execute TestNG tests in Eclipse IDE
without installing the TestNG plugin for Eclipse, this commit introduces
a JUnit Platform @Suite class that can be executed within the IDE.

See gh-27407

spring-test/src/test/java/org/springframework/test/context/testng/TestNGTestSuite.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2002-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.context.testng;
+
+import org.junit.platform.suite.api.IncludeEngines;
+import org.junit.platform.suite.api.SelectPackages;
+import org.junit.platform.suite.api.Suite;
+
+/**
+ * JUnit Platform based test suite for tests written in TestNG that involve the
+ * Spring TestContext Framework.
+ *
+ * <p><strong>This suite is only intended to be used manually within an IDE.</strong>
+ *
+ * <h3>Logging Configuration</h3>
+ *
+ * <p>In order for our log4j2 configuration to be used in an IDE, you must set the
+ * following system property before running any tests &mdash; for example, in
+ * <em>Run Configurations</em> in Eclipse.
+ *
+ * <pre style="code">
+ * -Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager
+ * </pre>
+ *
+ * @author Sam Brannen
+ * @since 5.3.11
+ */
+@Suite
+@IncludeEngines("testng")
+@SelectPackages("org.springframework.test.context.testng")
+class TestNGTestSuite {
+}