tarfs: Really prevent descending into a non-directory.

The previous fix was incorrect: we need to verify that the current node, if it exists, is not a directory, but we were checking the parent node instead. Address this, add more tests, and fix the test cleanup routines.

PR: 269519, 269561
Fixes: ae6cff89738b
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38645