Fix -Wunused-const-variable in descriptor_tests.cpp
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.
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