kyua: Try harder to delete directories.

When recursing into a directory to delete it, start by chmod'ing it to
0700. This fixes an issue where kyua is able to run, but not debug, a
test case that creates unwriteable directories, because when debugging
it tries (and fails) to delete the directory after the test completes.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: igoro
Differential Revision: https://reviews.freebsd.org/D51229