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/rightHandOverride.kt

@@ -0,0 +1,20 @@
+// Changed when traits were introduced. May not make sense any more
+
+interface Left {}
+open class Right() {
+    open fun f() = 42
+}
+
+class D() : Left, Right() {
+    override fun f() = 239
+}
+
+fun box() : String {
+    val r : Right = Right()
+    val d : D = D()
+
+    if (r.f() != 42) return "Fail #1"
+    if (d.f() != 239) return "Fail #2"
+
+    return "OK"
+}