Update 'Data-driven tests' design document (#807)

* Update tests design document

* Changes to 'test content' section

* Update exploration/data-driven-tests.md

Co-authored-by: Eemeli Aro <[email protected]>

* Apply suggestions from code review

Co-authored-by: Eemeli Aro <[email protected]>

* Delete sentence fragment

* Remove emphasis (consistent with PR suggestion)

---------

Co-authored-by: Eemeli Aro <[email protected]>

Author: mradbourne