test_util.py uses os.symlink which can fail on Windows

[jayvdb]

I am running unprivileged on a corporate laptop, and I encounter the following errors. It would be nice to detect this and skip the tests which depend on it.

FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_0_check_symlink - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_1_check_realpath - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_2_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_3_sideways_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_4_recursive_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_5_recursive_circular_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_6_detect_broken_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_7_ignore_broken_links - OSError: symbolic link privilege not held
FAILED pathspec/tests/test_util.py::IterTreeTest::test_2_8_no_follow_links - OSError: symbolic link privilege not held

[cpburnz]

Those tests will now be skipped when there's a lack of user permissions.