Kotlin test optional enum (#6201)

* Add test for optional enums in Kotlin

* Rename optional_scalars2.fbs into optional_scalars.fbs

Also updated all references in the project to point to
"optional_scalars.fbs" instead of "optional_scalars2.fbs".

tests/rust_usage_test/tests/optional_scalars_test.rs

@@ -1,5 +1,5 @@
 #[allow(dead_code, unused_imports)]
-#[path = "../../optional_scalars2_generated.rs"]
+#[path = "../../optional_scalars_generated.rs"]
 mod optional_scalars_generated;
 use crate::optional_scalars_generated::optional_scalars::*;