Add dependency on JUnit Platform Launcher for IDEA

This commit introduces an explicit dependency on the latest JUnit
Platform Launcher API so that it overrides whatever is bundled by
default in IntelliJ IDEA.

This allows a more seamless upgrade to newer versions of JUnit 5 and
the JUnit Platform before IDEA has been officially updated to support
those versions.

build.gradle

@@ -1084,6 +1084,9 @@ project("spring-test") {
 		testCompile('de.bechte.junit:junit-hierarchicalcontextrunner:4.12.1')
 		testCompile('io.projectreactor.ipc:reactor-netty')
 		testRuntime("org.junit.jupiter:junit-jupiter-engine:${junitJupiterVersion}")
+		// Pull in the latest Launcher API so that it overrides whatever
+		// is bundled by default in IntelliJ IDEA.
+		testRuntime("org.junit.platform:junit-platform-launcher:${junitPlatformVersion}")
 		testRuntime("org.apache.logging.log4j:log4j-jul:${log4jVersion}")  // Java Util Logging for JUnit 5
 		testRuntime("org.ehcache:ehcache:${ehcache3Version}")
 		testRuntime("org.terracotta:management-model:2.0.0")