test: add comment/no-comment cases to parse_array
test: add comment/no-comment cases to parse_array

When we add a macro to change the default comment preservation scheme,
some of the current tests that assume comments are discarded by defualt
fails. To make it more robust, we need to explicitly specify the comment
preservation scheme and add test cases for both of discard_ and
preserve_comments.

ToruNiina/toml11