Update tests to use Gerrit 2.8.6.1

Gerrit 2.8.6.1 bundles an updated mina-sshd which solves the
1-in-256 SSH hash mismatch bug, so should make our tests much more
stable.

Change-Id: Ic607660af8feb5c2dc4d1fca2681265eefdc7e93
Merge "Update tests to use Gerrit 2.8.6.1"