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/private/arrayConvention.kt

@@ -0,0 +1,21 @@
+var result = "fail"
+
+private operator fun X.get(name: String) = name + "K"
+private operator fun X.set(name: String, v: String) {
+    result = v
+}
+
+class X {
+    fun test() : String {
+        if (this["O"] != "OK") return "fail 1: ${this["O"]}"
+
+        this["O"] += "K"
+        if (result != "OKK") return "fail 2: ${result}"
+
+        return "OK"
+    }
+}
+
+fun box(): String {
+    return X().test()
+}

backend.native/tests/external/codegen/blackbox/private/kt9855.kt

@@ -0,0 +1,15 @@
+class MyString(var content : String)
+
+object Greeter {
+    fun sayHello(name : String): String {
+        var result = MyString(name)
+        result += "K"
+        return result.content
+    }
+}
+
+private operator fun MyString.plus(suffix: String) : MyString =  MyString("${this.content}$suffix")
+
+fun box(): String {
+    return Greeter.sayHello("O")
+}