[Tests] Add aidge_core.testing module and file tree utilities

Add a new aidge_core.testing module for factoring common
testing utilities for aidge_core and other aidge components
python unit tests.

Add a first contribution to the aidge_core.testing.utils
submodule for file tree management:
- tree_move()/tree_remove() for base file tree operations,
- tree_update_from_cache() for automatically benefit from
a file tree cache in particular when re-generating file trees
in tests on export. The main objective being to speedup tests.