make test_timersource to work on slower systems

:Release Notes:
Now test_timersource doesn't fail under valgrind (when running
ExperimentalMemCheck) on Intel Core i7-3770 CPU 3.40GHz

:Detailed Notes:
- ensured that timer fire may vary in range -50 .. +50 ms

:Testing Performed:
unit-tests on desktop

:QA Notes:

:Issues Addressed:
[BHV-1589] Implement category registration with its description

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

Change-Id: Ifadfe8770e10cf11924e22fba232dbfa53e43208
Reviewed-on: https://g2g.palm.com/5253
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>