Disable build of unit tests by default

:Release Notes:
Add an option WEBOS_CONFIG_BUILD_TESTS to control a build of unit tests

:Detailed Notes:

:Testing Performed:
build with enabled/disabled tests

:QA Notes:

:Issues Addressed:
[BHV-4671] Don't build unit tests of luna-service2 by default

Open-webOS-DCO-1.0-Signed-off-by: Alekseyev Oleksandr<alekseyev.oleksandr@lge.com>

Change-Id: Iea08d06bc16bf28b0cb92b7de49c1e09f0875b53
Reviewed-on: https://g2g.palm.com/5359
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>