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/strings/forInString.kt

@@ -0,0 +1,13 @@
+// WITH_RUNTIME
+
+fun foo(): Int {
+    var sum = 0
+    for (c in "239")
+        sum += (c.toInt() - '0'.toInt())
+    return sum
+}
+
+fun box(): String {
+    val f = foo()
+    return if (f == 14) "OK" else "Fail $f"
+}