[Build] Align build to new aidge_core dependencies

Context

Simplify dependencies for binding and allow both TEST and PYBIND on. This aligns to the new aidge_core dependencies which are self-sufficient.

This change requires the installed aidge_core library to include the merge request aidge_core!187 (merged)

Modified files

• CmakeLists.txt: remove useless dependencies on pybind and fmt, fix file name of export build file, allow both TEST and PYBIND
• aidge_backend_cpu-config.cmake.in: add missing dependency on aidge_core
• cmake/PybindModuleCreation.cmake: minor fixes
• setup.py: add envvar AIDGE_BUILD_TEST=OFF|ON for building C++ tests
• unit_tests/CMakeLists.txt: minor change, prefer to use PRIVATE when linking final exe/module/library (no behavioral impact)

TODO

• Need first to merge in aidge_core: aidge_core!187 (merged)