Add test of subscribing to PDM notifications

:Release Notes:
Add test for PDM subscribtion from DB8

:Detailed Notes:

:Testing Performed:

:QA Notes:

:Issues Addressed:

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>
Change-Id: I8108dcc926e32c79b0f965285e6264010536a9a0
Reviewed-on: https://g2g.palm.com/2989
Reviewed-by: Roger Stringer <roger.stringer@lge.com>
Tested-by: Roger Stringer <roger.stringer@lge.com>