TST: rewrite convert_dtypes test to make it easier extendable
I have a branch adding `convert_floating` support for ``convert_dtypes``, and the current way they are written is not really maintainable when adding more keywords (eg the tuples of 4 would become tuples of 5, making it even harder to know what is what, and making the formatting less easy to follow).

In this PR, I just refactored how the test cases are defined and didn't change any actual content.