[Cashtab] Better tests for opreturn target output generating functions

Summary: Unit tests should match the exact expected output. Neglected previously as did not realize could do it easily enough (while maintaining an easy-to-read hex string for the desired OP_RETURN) by generating the Buffer in the mocks.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16342