QLocalSocket benchmark: Report server error string if listen() fails

Including the error string gives whoever's running the test at least
some clue what's going wrong. One day it might even give them the
information they need to get later runs of the server past this
hurdle.

Task-number: QTBUG-95136
Change-Id: I5d67097339f1db78dfb7ba2ed4357121396977dd
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>

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

@@ -75,7 +75,9 @@ public:
             });
         });
 
-        QVERIFY(server.listen("foo"));
+        // TODO QTBUG-95136: on failure, remove the socket file and retry.
+        QVERIFY2(server.listen("foo"), qPrintable(server.errorString()));
+
         running.release();
         exec();
     }