Squish: Fix running tst_qtquick_creation3

...when running on build machine and VNC session has been started
by Jenkins. If so, the DISPLAY variable must be set automatically
and we should not override it.

Change-Id: Ic38267256c5ab8889d67fe6a4e7e46dba6515a67
Reviewed-by: Robert Loehning <robert.loehning@digia.com>