tests: use contains(QT_CONFIG,private_tests) consistently

tests: use contains(QT_CONFIG,private_tests) consistently

These tests used requires(contains(QT_CONFIG,private_tests)) in their
.pro file, but did not subtract themselves from their parent project
SUBDIRS when private_tests weren't enabled.

In the best case, this wastes a little time as qmake iterates over these
projects which won't be built.  In some worse esoteric cases, this may
break compilation or packaging.

Change-Id: If36b1b8f69c3509128786fec67899ae18ffaa2bc
Reviewed-by: Toby Tomkins <toby.tomkins@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>