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/unit/unitClassObject.kt

@@ -0,0 +1,11 @@
+fun box(): String {
+    Unit
+
+    val a = Unit
+    val b = Unit
+    if (a != b) return "Fail a != b"
+
+    if (Unit != Unit) return "Fail Unit != Unit"
+
+    return "OK"
+}