Decorate pagination 'next' and 'previous' buttons with data-test-subj

[justinkambic]

Summary

The purpose of this PR is to add data-test-subj to Pagination's next and previous buttons. I have a number of functional tests expecting a table's pagination to have these values, and when I replaced the existing table with eui components I got stuck. Link to those changes: elastic/kibana#22902

Screens of the props rendered in the docs:

I chose these values based on the existing naming used for the numbered buttons, per https://github.com/elastic/eui/blob/master/src/components/pagination/pagination.js#L33

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios

Note: anything unchecked here I did not feel was applicable, please let me know if I need to revisit any items

[chandlerprall]

LGTM, code review