Sindhu/path fix to run tf python unit tests

[avijit-nervana]

@crlishka We need to add better error checking in the test_ngtf.py. Something we will consider in a future PR.

[sindhu-nervana]

Hi @crlishka I made changes to exit with error when the test runner's test list is empty. This should throw error when test_ngtf.py fails to run run_tensorflow_pytests