Add unit tests for util functions

tests/test_util.py:
Add tests for `cwd()`, `natural_sort_path()`, `system()`,
`absolute_path()`, `transform_fd_to_tmpfile()`, `get_cert_paths()`,
`get_parent_cert_paths()` and `contains_fingerprint()`.

tests/test_util.py

+from os import chdir
+from os import getcwd
+from pathlib import Path
+from tempfile import NamedTemporaryFile
+from tempfile import TemporaryDirectory
+from typing import List
+from unittest.mock import Mock
+from unittest.mock import patch
+
+from pytest import mark
+from pytest import raises
+
+from libkeyringctl import util
+from libkeyringctl.types import Fingerprint
+
+
+def test_cwd() -> None:
+    with TemporaryDirectory() as tmp_dir_name:
+        tmp_dir = Path(tmp_dir_name)
+        test_dir = tmp_dir / "test"
+        test_dir.mkdir()
+        chdir(tmp_dir)
+        with util.cwd(new_dir=test_dir):
+            assert getcwd() == str(test_dir)
+        assert getcwd() == str(tmp_dir)
+
+
+@mark.parametrize(
+    "input_list, output_list",
+    [
+        ([Path("/foo"), Path("/bar/foo"), Path("/foo/20")], [Path("/foo/20"), Path("/foo"), Path("/bar/foo")]),
+    ],
+)
+def test_natural_sort_path(input_list: List[Path], output_list: List[Path]) -> None:
+    assert util.natural_sort_path(_list=input_list) == output_list
+
+
+@mark.parametrize(
+    "raise_on_cmd, exit_on_error",
+    [
+        (False, True),
+        (False, False),
+        (True, True),
+        (True, False),
+    ],
+)
+@patch("libkeyringctl.util.exit")
+@patch("libkeyringctl.util.check_output")
+def test_system(check_output_mock: Mock, exit_mock: Mock, raise_on_cmd: bool, exit_on_error: bool) -> None:
+    if raise_on_cmd:
+        check_output_mock.side_effect = util.CalledProcessError(returncode=1, cmd="foo", output="output")
+
+        with raises(util.CalledProcessError):
+            util.system(["foo"], exit_on_error=exit_on_error)
+            if exit_on_error:
+                exit_mock.assert_called_once_with(1)
+
+    else:
+        check_output_mock.return_value = b"output"
+        assert util.system(["foo"], exit_on_error=exit_on_error) == "output"
+
+
+def test_absolute_path() -> None:
+    with TemporaryDirectory() as tmp_dir_name:
+        tmp_dir = Path(tmp_dir_name)
+        test_dir = tmp_dir / "test"
+        test_dir.mkdir()
+        chdir(tmp_dir)
+        assert util.absolute_path(path="test") == test_dir
+
+
+def test_transform_fd_to_tmpfile() -> None:
+    with TemporaryDirectory() as tmp_dir_name:
+        tmp_dir = Path(tmp_dir_name)
+        with NamedTemporaryFile(dir=tmp_dir) as tmp_file:
+            tmp_file_fd = tmp_file.fileno()
+            util.transform_fd_to_tmpfile(
+                working_dir=tmp_dir,
+                sources=[Path("/foo"), Path(f"/proc/self/fd/{tmp_file_fd}")],
+            )
+
+
+def test_get_cert_paths() -> None:
+    with TemporaryDirectory() as tmp_dir_name:
+        tmp_dir = Path(tmp_dir_name)
+
+        cert_dir1 = tmp_dir / "cert1"
+        cert_dir1.mkdir()
+        cert1 = cert_dir1 / "cert1.asc"
+        cert1.touch()
+        cert_dir2 = tmp_dir / "cert2"
+        cert_dir2.mkdir()
+        cert2 = cert_dir2 / "cert2.asc"
+        cert2.touch()
+
+        assert util.get_cert_paths(paths=[tmp_dir]) == set([cert_dir1, cert_dir2])
+
+
+def test_get_parent_cert_paths() -> None:
+    with TemporaryDirectory() as tmp_dir_name:
+        tmp_dir = Path(tmp_dir_name)
+
+        keyring_dir = tmp_dir / "keyring"
+        group_dir = keyring_dir / "parent"
+        user_dir = group_dir / "parent"
+        cert_dir1 = user_dir / "cert1"
+        cert_dir1.mkdir(parents=True)
+        cert1 = cert_dir1 / "cert1.asc"
+        cert1.touch()
+        cert_dir2 = cert_dir1 / "cert2"
+        cert_dir2.mkdir(parents=True)
+        cert2 = cert_dir2 / "cert2.asc"
+        cert2.touch()
+
+        assert util.get_parent_cert_paths(paths=[cert1, cert2]) == set([cert_dir1])
+
+
+@mark.parametrize(
+    "fingerprints, fingerprint, result",
+    [
+        (
+            [Fingerprint("foo"), Fingerprint("bar")],
+            Fingerprint("foo"),
+            True,
+        ),
+        (
+            [Fingerprint("foo"), Fingerprint("bar")],
+            Fingerprint("baz"),
+            False,
+        ),
+    ],
+)
+def test_contains_fingerprint(fingerprints: List[Fingerprint], fingerprint: Fingerprint, result: bool) -> None:
+    assert util.contains_fingerprint(fingerprints=fingerprints, fingerprint=fingerprint) is result