Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/super/basicmethodSuperClass.kt

@@ -0,0 +1,17 @@
+import java.util.ArrayList
+
+class N() : ArrayList<Any>() {
+    override fun add(el: Any)  : Boolean {
+        if (!super<ArrayList>.add(el)) {
+            throw Exception()
+        }
+        return false
+    }
+}
+
+fun box(): String {
+    val n = N()
+    if (n.add("239")) return "fail"
+    if (n.get(0) == "239") return "OK";
+    return "fail";
+}