Write gst-validate scenarios for vaapi plugin elements

Submitted by Sreerenj Balachandran @sree

Link to original bug (#773304)

Description

We should write gst-validate test scenarios for the vaapi element regression testing. Either we can add them as part of gstremaer-vaapi or extend the gst-integration-testsuites with vaapi test cases.This bug is to track the development.

Please let us know if anyone interested to work with this :)