Merge #12447: test: Add missing signal.h header

Summary:
dd7e42cbb4 test: Add missing signal.h header (Wladimir J. van der Laan)

Pull request description:

util_tests.cpp needs to include the signal.h header on FreeBSD.

Reported by denis2342 on IRC.

Tree-SHA512: 0b946fc2770ca6043087f4fe22be96848d46efe44e3d4d45c5ee663e0f95b0e2e6145f4ade577a6257d0c0c4cfadf324111799e78b9e4e4be4da8a90f68e349f

Backport of Core PR12447
https://github.com/bitcoin/bitcoin/pull/12447/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3845