Refactor test infrastructure for metadata stress tests

This patchset adds a Python package in `utils/python` containing a common base class, `stress.MetadataTest`, to be used by all metadata stress tests. The class is responsible for parsing arguments, creating/removing directories and files, printing statuses, as well as opening and closing the metadata file, etc.

Another PR should take care of refactoring the stream stress tests, adding a common base class for both test types.