Improvements to pymarian tests

[snukky]

Proposed improvements:

1. Log "Using Python: " when running run_mrt.sh with a path and python version.
2. Log "Using pymarian: ".
3. Let overwrite the Python binary similarly to providing marian binary, for example PYTHONBIN=python3 run_mrt.sh
4. In tests/pymarian/setup.sh consider the case when multiple .whl files are present in marian's build/ directory and instead of failing, maybe pick just the first one.
5. The pymarian test should not fail due to not being able to download the licensed COMET-Kiwi model. I believe we can split test_pymarian.sh into 2 .sh test files: (1) one running COMET-Kiwi unit tests, which will be skipped if the model cannot be downloaded and 2) the other running all other unit tests.

[snukky]

The motivation for 5. is that one may not be able to download that model in automatic testing.

[snukky]

FYI, @thammegowda