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/kt889.kt

@@ -0,0 +1,12 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+operator fun Int.plus(s: String) : String {
+  System.out?.println("Int.plus(s: String) called")
+  return s
+}
+
+fun box() : String {
+   val s = "${1 + "a"}"
+   return if(s == "a") "OK" else "fail"
+}