Update to Kotlin 1.60 and Gradle 4.9

Removes the dependency on legacy script compilation and now relies on script
discovery.  In addition, the Gradle build scripts were refactored and updated
to be compatible with Gradle 5.0 and make use of the new java-library
configurations.

game/plugin-testing/build.gradle

@@ -1,13 +1,15 @@
-apply plugin: 'kotlin'
+plugins {
+    id 'java-library'
+    id 'org.jetbrains.kotlin.jvm'
+}
 
 dependencies {
-    compileOnly project(':game')
-    compile group: 'org.assertj', name: 'assertj-core', version: '3.8.0'
+    api project(':game')
+    api project(':net')
 
-    def gameTestConfiguration = project(':game').configurations.testCompile
-    def gameTestDependencies = gameTestConfiguration.dependencies
+    api group: 'junit', name: 'junit', version: junitVersion
+    api group: 'org.powermock', name: 'powermock-api-mockito', version: powermockVersion
+    api group: 'org.assertj', name: 'assertj-core', version: assertjVersion
 
-    gameTestDependencies.each {
-        compile it
-    }
+    implementation group: 'org.powermock', name: 'powermock-module-junit4', version: powermockVersion
 }