test_db_home_dir_online_backup CI test break other tests

[progier389]

Issue Description
test_db_home_dir_online_backup CI test change the dbhome to a directory that
is removed at the test completion but the dbhome is not restored.
So further testcases using the same topology fails

Instead of using tempfile.TemporaryDirectory as context manager we should use a more complex one
that also set/reset nsslapd-db-home-directory

Package Version and Platform:

• Platform: [e.g. Fedora] Fedora
• Package and version: main branch

Steps to Reproduce
Steps to reproduce the behavior:

1. Run backup CI tests on bdb
2. See error (test_after_db_log_rotation is failing)

Expected results
All tests should PASS