fix unit tests

:Release Notes:
Fix intermittent issues in tests caused by race between 3 different
threads.

:Detailed Notes:
- added explicit main-loop stop before LS::Service destructor

:Testing Performed:
Reasonable repeats of test_client integration test and other tests.

:QA Notes:

:Issues Addressed:
[BHV-7028] LS2: test_call still fails sometimes on TestClient.ServerStatus

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

Change-Id: If5c2c7d4b809817825814056db1fd0dcf4b2d431
Reviewed-on: https://g2g.palm.com/5561
Reviewed-by: DCO Verification
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Tested-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Reviewed-by: Alexandr Smoktal <alexandr.smoktal@lge.com>