Rewrite tcpSocketNonBlockingTest() and downloadBigFile() to use lambdas for
the slots. This allows for removing the related member variables and slots
of the test class and ensures no leaks of sockets or inconsistent values.
Add an error handler printing the error message to the flaky downloadBigFile()
test.
Change-Id: Ieb64063c41e045a1a50a6d074bef01753ee319ef
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
45948967bd