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/FlatBuffers.Test.Swift/SwiftTest.sh

@@ -4,7 +4,7 @@ test_dir=`pwd`
 
 cd ${swift_dir}/Tests/FlatBuffers.Test.SwiftTests
 ${test_dir}/../flatc --swift --gen-mutable --grpc --gen-object-api -I ${test_dir}/include_test ${test_dir}/monster_test.fbs ${test_dir}/union_vector/union_vector.fbs
-${test_dir}/../flatc --swift ${test_dir}/optional_scalars2.fbs
+${test_dir}/../flatc --swift ${test_dir}/optional_scalars.fbs
 cd ${swift_dir}
 swift build --build-tests
 swift test