Tiered tests helper functions

In the PR for WT-8272 donald.anderson suggested creating a helper function for tiered storage python tests for object naming, file naming, etc. This ticket is to do a slightly larger consideration of the different types of naming that tiered storage python tests do and add helper functions and use them when appropriate.

One example is:

  def tiered_object_name(uri, n):

     assert uri.startswith('table:')

     return uri[6:] + '-{:010d}'.format(n)