Remove 'tests' as default build part.

Remove 'tests' as default build part.

This requires 'configure -make tests' if you want to automatically
build autotests for a module by default.

You can still go into the individual tests/ directories and
    'qmake && make check'
to build and run the autotests.

configure -developer-build will enable the tests by default,
like it did in Qt4.

Change-Id: If4d870987de0947a8328509dcc227fa9e6284201
Reviewed-by: Jason McDonald <jason.mcdonald@nokia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>