tests: cql_query_test: Init local cache only once

It's a singleton, so we can't attempt to init it more than once.

Fixes cql_query_test failure:

/home/tgrabiec/src/urchin2/seastar/core/future.hh:315: void future_state<>::set(): Assertion `_u.st == state::future' failed.
unknown location(0): fatal error in "test_create_table_statement": signal: SIGABRT (application abort requested)
seastar/tests/test-utils.cc(31): last checkpoint

tests/cql_test_env.cc

@@ -201,6 +201,8 @@ future<> init_once(shared_ptr<distributed<database>> db) {
         new shared_ptr<distributed<database>>(db);
         return init_storage_service(*db).then([] {
             return init_ms_fd_gossiper("127.0.0.1", db::config::seed_provider_type());
+        }).then([] {
+            return db::system_keyspace::init_local_cache();
         });
     } else {
         return make_ready_future();
@@ -226,8 +228,6 @@ future<::shared_ptr<cql_test_env>> make_env_for_test() {
                 mm.start().get();
                 qp->start(std::ref(proxy), std::ref(*db)).get();
 
-                db::system_keyspace::init_local_cache().get();
-
                 auto& ss = service::get_local_storage_service();
                 static bool storage_service_started = false;
                 if (!storage_service_started) {