test: Replace remaining sprintf with snprintf

Summary:
Use of sprintf is seen as a red flag as many of its uses are insecure.
OpenBSD warns about it while compiling, and some modern platforms, e.g.
cloudlibc from cloudabi don't
even provide it anymore.

Backport of core PR9867

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2697