Fix avalanche test with boost 1.58

Summary:
D4055 introduced the timeout decorator which wasn't introduced until boost 1.59.0:
https://www.boost.org/doc/libs/1_58_0/libs/test/doc/html/boost_test/testing_tools/timeout.html
vs
https://www.boost.org/doc/libs/1_59_0/libs/test/doc/html/boost_test/testing_tools/timeout.html
(This is not explicitly called out in their release notes)

Considering that boost 1.58 is latest boost package maintained for Xenial (Ubuntu 16.04), I think we
should continue to support it until we've determined that we will explicitly no longer support it.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4098