Summary:
Use continents/countries in docrowwiseiterator test to make it easier to reason about.
Jira: DB-13690

Test Plan:
Jenkins: test regex: .*docrowwiseiterator-test.*

./yb_build.sh --cxx-test docrowwiseiterator-test

Reviewers: stiwary

Reviewed By: stiwary

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D39476