Adapt pytest so the CI run does not fail if IMGT.org is not available

[DarioMarzella]

Sometimes, imgt.org becomes unavailable for short periods of time.
In those cases, pytest will not be able to test the generation of PANDORA's database and it will result in a crashed CI run.

The test should be able to handle this situation, raise a warning and proceed without having the run crash.

[DarioMarzella]

It is actually better to leave the CI runs crash, so we make sure no errors about the IMGT database generation are ignored. This won't be fixed.