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/incDecOnObject.kt

@@ -0,0 +1,41 @@
+class X(var value: Long)
+
+operator fun X.inc(): X {
+    this.value++
+    return this
+}
+
+operator fun X.dec(): X {
+    this.value--
+    return this
+}
+
+class Z {
+
+    public var counter: Int = 0;
+
+    public var prop: X = X(0)
+        get()  {
+            counter++; return field
+        }
+        set(a: X) {
+            counter++
+            field = a;
+        }
+}
+
+fun box(): String {
+    var z = Z()
+    z.prop++
+
+    if (z.counter != 2) return "fail in postfix increment: ${z.counter} != 2"
+    if (z.prop.value != 1.toLong()) return "fail in postfix increment: ${z.prop.value} != 1"
+
+    z = Z()
+    z.prop--
+
+    if (z.counter != 2) return "fail in postfix decrement: ${z.counter} != 2"
+    if (z.prop.value != -1.toLong()) return "fail in postfix decrement: ${z.prop.value} != -1"
+
+    return "OK"
+}