Add unittests and related scripts

To run tests, navigate to Athos/tests directory (or provide path to pytest):
1. Run all tests with target as CPP. Backend can be CPP,3PC,2PC_HE,2PC_OT
   pytest -rs . --backend="CPP"
2. Run a specific test.
   pytest -rs . -k "test_arith_binop" --backend="CPP"
3. Run and generate a coverage report
   pytest --cov --cov-report html --cov-config=pytest_coverage_tf.config .

Install pytest and pytest-cov to run the above commands.