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

compiler/testData/codegen/classes/initializerBlockDImpl.kt

@@ -0,0 +1,20 @@
+import java.util.*
+import java.io.*
+
+class World() {
+  public val items: ArrayList<Item> = ArrayList<Item>()
+
+  inner class Item() {
+    {
+      items.add(this)
+    }
+  }
+
+  val foo = Item()
+}
+
+fun box() : String {
+  val w = World()
+  if (w.items.size() != 1) return "fail"
+  return "OK"
+}