build_db_cache() is used in tests to create test database templates, i.e.
DEFAULT_EMPTY_DB_ID (which is __default_empty_db__) and custom template
database using CubicwebTC test_db_id/pre_setup_database API.

When running tests in parallel using multiple processes, build_db_cache() may
try to build the same database twice. Avoid this by adding synchronisation of
process by using a file lock.

So when two processes require the same template database, one build the
database and others wait it to be created.

Use filelock (https://github.com/benediktschmitt/py-filelock) library to have a
portable (unix / windows) way for handling locks.
Also filelock is packaged in debian: https://packages.debian.org/source/python-filelock