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

compiler/testData/codegen/classes/propertyInInitializer.kt

@@ -0,0 +1,16 @@
+class Outer() {
+  val s = "xyzzy"
+
+  open inner class InnerBase(public val name: String) {
+  }
+
+  inner class InnerDerived(): InnerBase(s) {
+  }
+
+  val x = InnerDerived()
+}
+
+fun box() : String {
+  val o = Outer()
+  return if (o.x.name != "xyzzy") "fail" else "OK"
+}