Add comments to descriptor tests

Summary:
This is a partial backport of Core PR14646 : https://github.com/bitcoin/bitcoin/pull/14646/commits/26879509f1a3b9fc0fa616164e5a88fdb344ac4d

Depends on D6197

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6198