Make LearningTestTool Able to Use `khiops_env` if Requested

[popescu-v]

Description

Currently, kht_test.py in LearningTestTool/py does not use the settings in khiops_env. This entails that, in order to make sure that LearningTestTool tests Khiops in (real) user-like conditions, LearningTestTool needs to replicate khiops_env settings.

For example, to match the export MPICH__INTERFACE_HOSTNAME=localhost command in khiops_env on MacOS, kht_test.py needs to detect the "Darwin" platform and replicate khiops_env behavior by adding the -host localhost options to mpiexec, which achieve the same effect (related issue on this point: #401).

The goal of this issue is to make LearningTestTool launchable in two modes:

1. a "dev" mode, which keeps the current behavior; usable:
   • by Khiops core developers, and
   • in the standard CI tests which use the MODL* binaries directly
2. a "package" / "release" mode, which executes khiops_env --env to read the MPI command (and other settings); usable:
   • in the packaging CI tests (for all platforms)
   • in the LearningTest full tests.

Context

• Khiops version: >= 10.2.3
• Scenario file (use khiops -o scenario._kh): Any
• OS description (use khiops -s): All supported

[popescu-v]

It suffices to add a --environment option that gives the path to the desired khiops_env.

[marcboulle]

Alternative simple: réutiliser le paramètre existant binaries pour y cherche le khiops_env en priorité, et si non trouvé se rabattre sur la recherche des binaires comme actuellement