repo_management/files.py:
Add iterator method `_read_db_file_member()` to yield the members of a
provided tarfile.TarFile.
The members are filtered by name using a regular expression, which can
be overridden.

tests/test_file.py:
Change `create_db_file()` to also create 'desc' and 'files' files below
the created temporary directories.
Assert, that all tested methods return a value.
Add test for `_read_db_file_member()`.