Abstract out IsSolvable from Witnessifier

Summary:
This is extracted from Core PR11403 : https://github.com/bitcoin/bitcoin/pull/11403/commits/0c8ea6380c9f402ed9777fd015b117ba13125a35

We don't have a witnessifier, so the usefulness is limited for us, but it'll help backport the relevent part of PR11403 and we can cleanup after the fact.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3121