Adding three tests to class.jetpack for sort_modules

The main Jetpack class was very lightly tested, so this
commit adds three tests covering the sort_modules class method,
which is both simple and would cause trouble if unintentionally
changed. I did not include a @since tag as I do not know when the
method was introduced.
Merge pull request #959 from enkrates/add-tests-to-jetpack-class

Adding three tests to class.jetpack for sort_modules