QLocalSocket benchmark: improve connectivity and error reporting

For nonblocking Unix domain sockets the connection may not be
completed immediately. So, add a blocking call to waitForConnected()
to improve test stability. Also, explain a possible reason that
cause the connection to fail on Unix.

Task-number: QTBUG-91713
Change-Id: If34070f2383fd0c854e2707c734fe5da4bda1b42
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

tests/benchmarks/network/socket/qlocalsocket/tst_qlocalsocket.cpp

@@ -116,6 +116,10 @@ public:
             });
 
             socket->connectToServer("foo");
+            QVERIFY2(socket->waitForConnected(), "The system is probably reaching the maximum "
+                                                 "number of open file descriptors. On Unix, "
+                                                 "try to increase the limit with 'ulimit -n 32000' "
+                                                 "and run the test again.");
             QCOMPARE(socket->state(), QLocalSocket::ConnectedState);
         }
     }