[backport#14820] test: Fix descriptor_tests not checking ToString output of public descriptors

Summary:
c77f09230b Fix descriptor_tests not checking ToString output of public descriptors (Russell Yanofsky)

Pull request description:

This fixes a minor test bug introduced in #13697 that I noticed while reviewing #14646

Tree-SHA512: efed91200cdff5f86ba5de3461ac00759d285e2905f6cb24cea15d3e23e0581ce5fc14b24a40db093f7ebd662ee1ee2cf67f8798bac1903a78298eda08909cfb

This is a backport of Core PR14820

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6149