testlib: fix compile error with macosx10.8

[NSDate date] returns an id, so one needs to send a message instead of
accessing a property.

Backport commit 92b3397a

 from the 5.5 branch.

Change-Id: Id70915e1ac23994a081765e0a527802fef61b573
Reviewed-by: Tim Blechmann <tim@klingt.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>