Minor updates:

• rename test_aby3.py to aby3_test.py to match rest of code base
• consider using np.testing.assert_allclose(x, x, rtol=0.0, atol=threshold) instead of custom close function (see https://github.com/tf-encrypted/tf-encrypted/pull/721/files#r362792674)
• consider using unittest.main() at the end of the scripts instead of re-listing all test functions (which could potentially become unsynchronised in the future); see eg pond_test.py