tests: Stop gossip during shutdown in cql_test_env

Fixes the heap-use-after-free error in build/debug/tests/auth_test

==1415==ERROR: AddressSanitizer: heap-use-after-free on address
0x62200032cfa8 at pc 0x00000350701d bp 0x7fec96df8d40 sp
0x7fec96df8d30
READ of size 8 at 0x62200032cfa8 thread T1
    #0 0x350701c in
_ZZN3gms8gossiper3runEvENKUlOT_E0_clI6futureIJEEEEDaS2_
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x350701c)
    #1 0x35795b1 in apply<gms::gossiper::run()::<lambda(auto:40&&)>,
future<> > /home/penberg/scylla/seastar/core/future.hh:1203
    #2 0x369103d in
_ZZN6futureIJEE12then_wrappedIZN3gms8gossiper3runEvEUlOT_E0_S0_EET0_S5_ENUlS5_E_clI12future_stateIJEEEEDaS5_
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x369103d)
    #3 0x369182a in run /home/penberg/scylla/seastar/core/future.hh:399
    #4 0x435f24 in
reactor::run_tasks(circular_buffer<std::unique_ptr<task,
std::default_delete<task> >, std::allocator<std::unique_ptr<task,
std::default_delete<task> > > >&) core/reactor.cc:1368
    #5 0x43a44f in reactor::run() core/reactor.cc:1672
    #6 0x952e4b in app_template::run_deprecated(int, char**,
std::function<void ()>&&) core/app-template.cc:123
    #7 0x58dc79d in test_runner::start(int,
char**)::{lambda()#1}::operator()()
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x58dc79d)
    #8 0x58e6cd6 in _M_invoke /usr/include/c++/5.3.1/functional:1871
    #9 0x688639 in std::function<void ()>::operator()() const
/usr/include/c++/5.3.1/functional:2271
    #10 0x8d939c in posix_thread::start_routine(void*) core/posix.cc:51
    #11 0x7feca02a4609 in start_thread (/lib64/libpthread.so.0+0x7609)
    #12 0x7fec9ffdea4c in clone (/lib64/libc.so.6+0x102a4c)

0x62200032cfa8 is located 5800 bytes inside of 5808-byte region
[0x62200032b900,0x62200032cfb0)
freed by thread T1 here:
    #0 0x7feca4f76472 in operator delete(void*, unsigned long)
(/lib64/libasan.so.2+0x9a472)
    #1 0x3740772 in gms::gossiper::~gossiper()
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x3740772)
    #2 0x2588ba1 in shared_ptr<gms::gossiper>::~shared_ptr()
seastar/core/shared_ptr.hh:389
    #3 0x4fc908c in
seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1}::~stop()
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x4fc908c)
    #4 0x4ff722a in future<>
future<>::then<seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1}, future<>
>(seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1}&&)::{lambda(seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1})#1}::~then()
(/home/penberg/scylla/build/debug/tests/auth_test_g+0x4ff722a)
    #5 0x509a28c in continuation<future<>
future<>::then<seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1}, future<>
>(seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1}&&)::{lambda(seastar::sharded<gms::gossiper>::stop()::{lambda(unsigned
int)#1}::operator()(unsigned
int)::{lambda()#1}::operator()()::{lambda()#1})#1}>::~continuation()
seastar/core/future.hh:395
    #6 0x509a40d in continuation<future<>
Message-Id: <f8f1c92c1eb88687ab0534f5e7874d53050a5b93.1453446350.git.asias@scylladb.com>

tests/cql_test_env.cc

@@ -320,6 +320,9 @@ class single_node_cql_env : public cql_test_env {
 
     virtual future<> stop() override {
         return seastar::async([this] {
+            // Started by storage_service::init_server()
+            gms::get_local_gossiper().stop_gossiping().get();
+
             _core_local.stop().get();
             db::system_keyspace::deinit_local_cache().get();