test: Pin nethsm container image version to c16fe4ed

We are experiencing issues with uploading of update files never returning. This is due to us reusing nethsm-py-sdk's dummy update file, which is now too old to be able to update the container. Changes to the tagging scheme, as well as a central, versioned location for testing update files have been requested upstream in https://github.com/Nitrokey/nethsm/issues/8

Fixes #32 (closed)

Signed-off-by: David Runge dvzrv@archlinux.org