This fixes gossip test shutdown similar to what commit 13ce48e ("tests:
Fix stop of storage_service in cql_test_env") did for CQL tests:
gossip_test: /home/penberg/scylla/seastar/core/sharded.hh:439: Service& seastar::sharded<Service>::local() [with Service = net::messaging_service]: Assertion `local_is_initialized()' failed.
Running 1 test case...
[snip]
unknown location(0): fatal error in "test_boot_shutdown": signal: SIGABRT (application abort requested)
seastar/tests/test-utils.cc(32): last checkpoint
Message-Id: <1458126520-20025-1-git-send-email-penberg@scylladb.com>
(cherry picked from commit 2f519b9b34)