Add a `binaries` marker to pytests

[gregtatum]

We have tests that require external binaries like Marian and others. These are all available in our local Docker images. However this image is quite slow on non-x86 systems. In this case it would be nice to mark tests that are requiring external binaries.

We can do this through pytest markers:

https://docs.pytest.org/en/7.1.x/example/markers.html

@pytest.mark.binaries

Then you can run them using -m "not binaries".