Build: reworked unit tests to use gtest in a more generic way

Build: reworked unit tests to use gtest in a more generic way

The benefit of this change is to keep most gtest related configuration
 in a single file and avoid to duplicate paths in sub sconfiles.

For portability concern "os.path.join" is preferred
 to appending unix paths.

Some libs are reordered and some cleanup was done.

Let's also remove hardcoded pathes, as there is not guarantee
 that the files will remain at the same place in all configurations.

Problem was observed when crossbuilding with Yocto distro
 with security checks enabled (LfAgl).

More refactoring and deeper cleanup might come in later patch.

Change-Id: Ia13ee8c03f65f3407e88ff21d51b292261c9f2a3
Bug: https://jira.iotivity.org/browse/IOT-863
Bug-AGL: https://jira.automotivelinux.org/browse/SPEC-158
Origin: https://github.com/tizenteam/iotivity/
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/8127
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: David Antler <david.a.antler@intel.com>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>
Reviewed-by: Jon A. Cruz <jon@joncruz.org>