Mute self assign warning in uint256_tests.cpp

Summary: While it is generaly a bad idea to do so, we want to test that the code behave properly if someone came to do this anyways. So we remove the warning using a noop.

Test Plan:

ninja check

Verify that the warnings are gone.

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5310