Singularity build scripts are broken

[ziotom78]

The Singularity images produced by the scripts in /singularity do no longer work for a combination of problems:

1. Some new tests save temporary files in the current directory instead of correctly relying on pytest to get a temporary file path
2. Some tests require the user to have a working IMO
3. singularity test does not work
4. import litebird_sim as lbs from the Python prompt no longer finds the package