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/traits/kt2260.kt

@@ -0,0 +1,9 @@
+interface Flusher {
+    fun flush() = "OK"
+}
+
+fun myFlusher() = object : Flusher { }
+
+fun flushIt(flusher: Flusher) = flusher.flush()
+
+fun box() = flushIt(myFlusher())