Add tests for InferDescriptor and Descriptor::IsSolvable

Summary:
This is a partial backport of Core PR14477 : https://github.com/bitcoin/bitcoin/pull/14477/commits/9b2a25b13f81a45ff59a6a4adde595404a6c062a

Depends on D6124 and D6125

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6126