Fix -Wunused-const-variable in descriptor_tests.cpp

Summary:
The UNSOLVABLE constant is never used, due the differences in out
codebase vs core.
Keeping it commented out to avoid reusing the value and allow for
resurrecting it if needed.

Test Plan:

ninja check

Ensure there is no -Wunused-const-variable warning in the output.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4622