--no-overwrite-dir no overwrite even temporarily

--no-overwrite-dir no overwrite even temporarily

Problem and fix reported by Pavel Cahyna in
https://lists.gnu.org/r/bug-tar/2025-01/msg00000.html
* src/extract.c (extract_dir): With --no-overwrite-dir,
skip the chmod if the directory already exists.
* tests/extrac23.at (--no-overwrite-dir on empty directory):
Move the part of the test that looks at a nonempty directory ...
* tests/extrac30.at: ... to this new file, because the test now
must be run as non-root.  Adjust the test to match the new behavior.
* tests/Makefile.am (TESTSUITE_AT), tests/testsuite.at: Add it.