Remove insignification from QtConcurrentFilter test.

This test has one unstable failure that occurs about 0.8% of the time in
CI.  Make the test catch this particular failure and re-enable the rest
of the test.

Task-number: QTBUG-20688
Change-Id: I22f5d372db77ea35163f96195ac0c174d8078899
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>

tests/auto/concurrent/qtconcurrentfilter/qtconcurrentfilter.pro

@@ -3,5 +3,3 @@ TARGET = tst_qtconcurrentfilter
 QT = core testlib concurrent
 SOURCES = tst_qtconcurrentfilter.cpp
 DEFINES += QT_STRICT_ITERATORS
-
-CONFIG += insignificant_test # See QTBUG-20688

tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp

@@ -1472,6 +1472,8 @@ void tst_QtConcurrentFilter::noDetach()
         QVERIFY(ll.isDetached() == false);
 
         QtConcurrent::filter(l, waitFilterfn).waitForFinished();
+        if (!l.isDetached())
+            QEXPECT_FAIL("", "QTBUG-20688: Known unstable failure", Abort);
         QVERIFY(l.isDetached() == true);
         QVERIFY(ll.isDetached() == true);
     }