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