[test] add a couple test cases to uint256_tests.cpp

Summary:
make sure template <unsigned int BITS> void base_blob<BITS>::SetHex(const char *psz) properly skips
leading spaces and 0x prefixes

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6323