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/operatorConventions/nestedMaps.kt

@@ -0,0 +1,13 @@
+object Map1 {
+    operator fun get(i: Int, j: Int) = Map2
+}
+
+object Map2 {
+    operator fun get(i: Int, j: Int) = 0
+    operator fun set(i: Int, j: Int, newValue: Int) {}
+}
+
+fun box(): String {
+    Map1[0, 0][0, 0]++
+    return "OK"
+}