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/stringPlusOnlyWorksOnString.kt

@@ -0,0 +1,7 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    var x: MutableCollection<Int> = ArrayList()
+    x + ArrayList()
+    return "OK"
+}