tst_qtcpsocket: fix comment, the slot goes to 512

Change-Id: Ia0c5b29d6e02c9fda0b1da4a2779f1cbe9b1d747
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Doc: QAtomicInteger first appeared in Qt 5.3

Change-Id: I900e5b0ec8291d34685cb545540a5a9f54551d05
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>