Fix a copy in sigencoding_tests

Summary:
The same one gets fixed in D3796, but there were 2 occurrences in the
file.

Test Plan:
With Clang:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3797