Skip tests if backend device is not found

[diptorupd]

Currently we just check if a GPU is available. The check is not sufficient as it can cause an issue if the GPU is Level-0 and we are trying to run an OpenCL specific test.

[diptorupd]

@PokhodenkoSA I added changes for this issue in #93. Can you review the test cases with the changes I made to to check for right back end before running the test case?

[diptorupd]

Done as part of #93