tests: switch base64 to secretbase

[shikokuchuo]

Prework

• [x] I understand and agree to the Contributor Code of Conduct.
• [ ] I have already submitted a discussion topic or issue to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: #

Summary

There is a test that was using nanonext::base64().

Base64 is being retired from nanonext as the final part of moving out general utility functions, refocusing it more tightly on being a communications package.

These functions were added recently to v1.0.0 of secretbase, completing the scope of that package.

This PR fixes the test, but takes in secretbase as a soft dependency. Feel free, if you wish, to not merge this PR but modify the test to use something else instead.

[wlandau]

Thanks for the FYI, fixed in fc5fe45b3024b9a4dd6a7ca1645b9573c2ac4e2e