Add expected failures for timestamp tests

They started to fail since Qt 5.11:

    FAIL!  : Cli7zTest::testList(normal-file-9381) Compared values are not the same
       Actual   (entry->property("timestamp").toString()): "2015-05-17T19:41:48.000"
       Expected (expectedTimestamp)                      : "2015-05-17T19:41:48"

Something about datetime formats probably changed in Qt 5.11.

Task: T8144