Updated copy_wiredtiger_home test helper to copy bucket directory

The copy_wiredtiger_home test helper function, in helper.py copies the files in the WiredTiger directory to another directory.

Python tests that use tiered storage (including those executed via hook_tiered.py) create a bucket directory in the WiredTiger directory. copy_wiredtiger_home only copies files and therefore won't copy the bucket directory and its contents. Therefore a test that attempts to restart in the copied directory will typically fail when it can't find the bucket directory.

We should updated copy_wiredtiger_home to also copy the bucket directory and its contents.

Tiered storage also creates a cache directory. We should consider whether that needs to be copied also.