Migrate to TestNG Engine for the JUnit Platform in spring-test

Prior to this commit, we had configured separate test tasks for JUnit
and TestNG. In addition, we configured a standard `test` task that
depended on the `junit` and `testNG` tasks, and we had an additional
`aggregateTestReports` task that aggregated the reports from the JUnit
and TestNG test tasks.

Thanks to the introduction of the "TestNG Engine for the JUnit
Platform", this commit simplifies our Gradle build in the spring-test
module by running JUnit 4, JUnit Jupiter, and TestNG tests on the JUnit
Platform in a single Gradle `test` task.

See gh-27406

build.gradle

@@ -188,6 +188,7 @@ configure(allprojects) { project ->
 				exclude group: "junit", name: "junit"
 			}
 			dependency "org.testng:testng:7.4.0"
+			dependency "org.junit.support:testng-engine:1.0.1"
 			dependency "org.hamcrest:hamcrest:2.1"
 			dependency "org.awaitility:awaitility:3.1.6"
 			dependency "org.assertj:assertj-core:3.20.2"

spring-test/spring-test.gradle

@@ -77,43 +77,23 @@ dependencies {
 	testRuntimeOnly("org.junit.vintage:junit-vintage-engine") {
 		exclude group: "junit", module: "junit"
 	}
+	testRuntimeOnly("org.junit.support:testng-engine")
 	testRuntimeOnly("org.glassfish:javax.el")
 	testRuntimeOnly("com.sun.xml.bind:jaxb-core")
 	testRuntimeOnly("com.sun.xml.bind:jaxb-impl")
 }
 
-task junit(type: Test) {
-	description = "Runs JUnit 4 and JUnit Jupiter tests."
+test {
+	description = "Runs JUnit 4, JUnit Jupiter, and TestNG tests."
 	useJUnitPlatform {
+		includeEngines "junit-vintage", "junit-jupiter", "testng"
 		excludeTags "failing-test-case"
 	}
+	// We use `include` instead of `filter.includeTestsMatching`, since
+	// the latter results in some tests being executed/reported
+	// multiple times.
 	include(["**/*Tests.class", "**/*Test.class"])
-	exclude(["**/testng/**/*.*"])
 	systemProperty("testGroups", project.properties.get("testGroups"))
 	// Java Util Logging for the JUnit Platform.
 	// systemProperty("java.util.logging.manager", "org.apache.logging.log4j.jul.LogManager")
 }
-
-task testNG(type: Test) {
-	description = "Runs TestNG tests."
-	useTestNG()
-	include(["**/testng/**/*Tests.class", "**/testng/**/*Test.class"])
-	systemProperty("testGroups", project.properties.get("testGroups"))
-	// Show STD_OUT & STD_ERR of the test JVM(s) on the console:
-	// testLogging.showStandardStreams = true
-	// forkEvery 1
-}
-
-test {
-	description = "Runs all tests."
-	dependsOn junit, testNG
-	exclude(["**/*.*"])
-}
-
-task aggregateTestReports(type: TestReport) {
-	description = "Aggregates JUnit and TestNG test reports."
-	destinationDir = test.reports.html.outputLocation.get().getAsFile()
-	reportOn junit, testNG
-}
-
-check.dependsOn aggregateTestReports