Big overhaul to the way classpath setup works in this plugin

* Instead of trying to replicate whatever magic the Android Plugin applies,
simply leverage its power & share the classpath of the default unit tests
* Moved away from JUnit 5's Gradle Plugin a little more by removing
the inheritance from its Java-based plugin
* Moved setup of the JUnit 5 test tasks to their own class, using an
initialization approach not unlike the AGP's, with ConfigAction
* Repackaged classes

Author: Marcel Schnelle

android-junit5/build.gradle

@@ -50,6 +50,7 @@ dependencies {
     testCompile "junit:junit:$JUNIT4_VERSION"
     testCompile("org.spockframework:spock-core:$SPOCK_VERSION") { transitive = false }
 
+    testCompileOnly "com.android.tools.build:gradle:$ANDROID_PLUGIN_VERSION_3X"
     testAgp2xCompile "com.android.tools.build:gradle:$ANDROID_PLUGIN_VERSION_2X"
     testAgp3xCompile "com.android.tools.build:gradle:$ANDROID_PLUGIN_VERSION_3X"
 }