tst_QUdpSocket - remove insignificance

This is the second attempt to remove 'insignificant' from UDP socket test to:
- make changes ending up in regressions more difficult (hopefully impossible)  to merge
- switch to BLACKLIST if needed
- make flaky tests more visible.

For now this 'back to significant' will be accompanied by extended BLACKLIST.
New in BLACKLIST:
* OS X - datagram size-related problems (fixed in 5.7)
* OS X - multicastLeaveAfterClose - will probably stay BLACKLISTED,
  seems to be a Darwin's quirk, can be fixed in OS X >=  10.10.
* windows ...

tst_QUdpSocket::echo seems to fail randomly on OS X/linux and
it looks like it fails at the same time on different machines,
should be something server-related (a guess only).

Change-Id: Ib344348ffab03fab1b9309b80449a04d8ce247c6
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

tests/auto/network/socket/qudpsocket/BLACKLIST

@@ -1,8 +1,24 @@
 [multicast:same bind, group ipv6 address]
 *
-[multicast:valid bind, group ipv6 address]
+[multicast]
 osx
-[multicast:dual bind, group ipv6 address]
+[writeDatagramToNonExistingPeer]
+windows
 osx
-[multicast:same bind, group ipv4 address]
+[asyncReadDatagram]
+windows
 osx
+[multicastLeaveAfterClose]
+osx
+[readyRead]
+osx
+[readyReadForEmptyDatagram]
+osx
+[echo]
+linux
+osx
+[ipv6Loop]
+osx
+[loop]
+osx
+

tests/auto/network/socket/qudpsocket/test/test.pro

@@ -22,5 +22,3 @@ wince* {
 }
 
 TARGET = tst_qudpsocket
-
-CONFIG+=insignificant_test # QTBUG-25367, QTBUG-25368