backend/tests: Add blackbox tests from Kotlin JVM

Added tests from testData/codegen/box directory. There are blackbox tests
in other directories and they are to be added.

backend.native/tests/external/codegen/blackbox/classes/overloadPlusAssign.kt

@@ -0,0 +1,24 @@
+class ArrayWrapper<T>() {
+    val contents = ArrayList<T>()
+
+    fun add(item: T) {
+        contents.add(item)
+    }
+
+    operator fun plusAssign(rhs: ArrayWrapper<T>) {
+        contents.addAll(rhs.contents)
+    }
+
+    operator fun get(index: Int): T {
+        return contents.get(index)!!
+    }
+}
+
+fun box(): String {
+    var v1 = ArrayWrapper<String>()
+    val v2 = ArrayWrapper<String>()
+    v1.add("foo")
+    v2.add("bar")
+    v1 += v2
+    return if (v1.contents.size == 2) "OK" else "fail"
+}