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

compiler/testData/codegen/classes/simpleBox.kt

@@ -0,0 +1,8 @@
+class Box<T>(t: T) {
+    var value = t
+}
+
+fun box(): String {
+    val box: Box<Int> = Box<Int>(1)
+    return if (box.value == 1) "OK" else "fail"
+}