Add explicit start to servers

Reviewers: teon.banek

Reviewed By: teon.banek

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1661

tests/concurrent/network_read_hang.cpp

@@ -67,6 +67,7 @@ TEST(Network, SocketReadHangOnConcurrentConnections) {
   communication::ServerContext context;
   communication::Server<TestSession, TestData> server(endpoint, &data, &context,
                                                       -1, "Test", N);
+  ASSERT_TRUE(server.Start());
 
   const auto &ep = server.endpoint();
   // start clients

tests/concurrent/network_server.cpp

@@ -23,6 +23,7 @@ TEST(Network, Server) {
   int N = (std::thread::hardware_concurrency() + 1) / 2;
   ContextT context;
   ServerT server(endpoint, &session_data, &context, -1, "Test", N);
+  ASSERT_TRUE(server.Start());
 
   const auto &ep = server.endpoint();
   // start clients

tests/concurrent/network_session_leak.cpp

@@ -24,6 +24,7 @@ TEST(Network, SessionLeak) {
   TestData session_data;
   ContextT context;
   ServerT server(endpoint, &session_data, &context, -1, "Test", 2);
+  ASSERT_TRUE(server.Start());
 
   // start clients
   int N = 50;