Use Gradles built-in JUnit 5 support

game/plugin-testing/build.gradle

@@ -2,6 +2,8 @@ apply plugin: 'java-library'
 apply plugin: 'org.jetbrains.kotlin.jvm'
 
 dependencies {
+    test.useJUnitPlatform()
+
     api project(':game')
     api project(':net')
 

game/plugin/build.gradle

@@ -1,23 +1,9 @@
-buildscript {
-    repositories {
-        jcenter()
-        mavenCentral()
-
-        maven { url "https://plugins.gradle.org/m2/" }
-    }
-
-    dependencies {
-        classpath 'org.junit.platform:junit-platform-gradle-plugin:1.1.0'
-    }
-}
-
 subprojects { subproj ->
     if (subproj.buildFile.exists()) {
         apply plugin: 'apollo-plugin'
-        apply plugin: 'org.junit.platform.gradle.plugin'
-
 
         dependencies {
+            test.useJUnitPlatform()
             implementation group: 'com.google.guava', name: 'guava', version: guavaVersion
         }
     }