Resolve "Path directory prefix bypass in ubifs/output.py"

[qkaiser]

using path.startswith(basedir) without a trailing directory separator allows directory traversal if a target directory shares a common prefix.

For example, if basedir is /tmp/target, and path is ../target_hack, os.path.realpath resolves it to /tmp/target_hack, which passes the startswith check and allows writing outside the base directory.

Fixed by using the standardized implementation we use in jefferson. Implemented the test suite we use in unblob, and made sure we run the tests in CI.

Fixes #139

[elektrischermoench]

Just worth mentioning test is passing on old code also.

[elektrischermoench]

The fix is correct, but the test suite doesn't cover all effects of the bug. The old code accepted out_hack just because the name starts with out:

.venv/bin/python -m pytest tests/test_ubifs_output.py -v --no-cov
================================== test session starts ===================================
platform linux -- Python 3.14.7, pytest-9.1.1, pluggy-1.6.0 -- /tmp/claude-1000/-home-kai-development/281967f1-1e69-4fdb-beee-4285ab125719/scratchpad/ubi_reader/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/claude-1000/-home-kai-development/281967f1-1e69-4fdb-beee-4285ab125719/scratchpad/ubi_reader
configfile: pyproject.toml
plugins: cov-7.1.0
collected 11 items

tests/test_ubifs_output.py::test_is_safe_path[/lib/out-/lib/out/file-True] PASSED  [  9%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-file-True] PASSED           [ 18%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-dir/file-True] PASSED       [ 27%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/file-True] PASSED  [ 36%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/../file-True] PASSED [ 45%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/../../file-True] PASSED [ 54%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/../../../file-False] PASSED [ 63%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/../../../-False] PASSED [ 72%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-some/dir/../../..-False] PASSED [ 81%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-../file-False] PASSED       [ 90%]
tests/test_ubifs_output.py::test_is_safe_path[/lib/out-/lib/out/../file-False] PASSED [100%]

=================================== 11 passed in 0.09s ===================================

Maybe add tests like this:

        ("/lib/out", "../out_hack", False),
        ("/lib/out", "../outx", False),
        ("/lib/out", "../out.bak/file", False),
        ("/lib/out", "/lib/out_hack", False),
        ("/lib/out", "/lib/outfile", False),
        ("/lib/out", "some/dir/../../../out_hack/file", False),