Execute test suite in Gradle using JUnit Platform

Closes gh-23286

build.gradle

@@ -76,6 +76,9 @@ configure(allprojects) { project ->
 		generatedPomCustomization {
 			enabled = false
 		}
+		imports {
+			mavenBom "org.junit:junit-bom:${junit5Version}"
+		}
 	}
 
 	configurations.all {
@@ -134,6 +137,7 @@ configure(allprojects) { project ->
 		systemProperty("java.awt.headless", "true")
 		systemProperty("testGroups", project.properties.get("testGroups"))
 		systemProperty("io.netty.leakDetection.level", "paranoid")
+		useJUnitPlatform()
 		scanForTestClasses = false
 		include(["**/*Tests.class", "**/*Test.class"])
 		// Since we set scanForTestClasses to false, we need to filter out inner
@@ -165,6 +169,10 @@ configure(allprojects) { project ->
 		testCompile("io.mockk:mockk:1.9.3")
 		testCompile("org.hamcrest:hamcrest-all:1.3")
 		testCompile("org.assertj:assertj-core:3.12.2")
+		// Pull in the latest JUnit 5 Launcher API and the Vintage engine as well
+		// so that we can run JUnit 4 tests in IDEs.
+		testRuntime("org.junit.platform:junit-platform-launcher")
+		testRuntime("org.junit.vintage:junit-vintage-engine")
 		testRuntime("org.apache.logging.log4j:log4j-core:${log4jVersion}")
 		testRuntime("org.apache.logging.log4j:log4j-slf4j-impl:${log4jVersion}")
 		testRuntime("org.apache.logging.log4j:log4j-jul:${log4jVersion}")

spring-test/spring-test.gradle

@@ -79,11 +79,7 @@ dependencies {
 	}
 	testCompile("io.projectreactor.netty:reactor-netty")
 	testCompile("de.bechte.junit:junit-hierarchicalcontextrunner:4.12.1")
-	// Pull in the latest JUnit 5 Launcher API and the Vintage engine as well
-	// so that we can run JUnit 4 tests in IDEs.
 	testRuntime("org.junit.jupiter:junit-jupiter-engine")
-	testRuntime("org.junit.platform:junit-platform-launcher")
-	testRuntime("org.junit.vintage:junit-vintage-engine")
 	testRuntime("org.glassfish:javax.el:3.0.1-b08")
 	testRuntime("com.sun.xml.bind:jaxb-core:2.3.0.1")
 	testRuntime("com.sun.xml.bind:jaxb-impl:2.3.0.1")