More robust symlink creation test

The current test suite fails when running on a new enough Python version
on Windows to have the `os.symlink` function available (Python 3.2 and
later), but without privileges to create symlinks, which is the general
case when not running as administrator.

This fix skips the symlink creation tests when the test process either
does not have symlink capabilities, or has them but not enough privilege
to create symlinks.

dulwich/tests/test_index.py

@@ -68,6 +68,25 @@ from dulwich.tests.utils import (
     )
 
 
+def can_symlink():
+    """Return whether running process can create symlinks."""
+    if sys.platform != 'win32':
+        # Platforms other than Windows should allow symlinks without issues.
+        return True
+
+    if not hasattr(os, 'symlink'):
+        # Older Python versions do not have `os.symlink` on Windows.
+        return False
+
+    test_source = tempfile.mkdtemp()
+    test_target = test_source + 'can_symlink'
+    try:
+        os.symlink(test_source, test_target)
+    except OSError:
+        return False
+    return True
+
+
 class IndexTestCase(TestCase):
 
     datadir = os.path.join(os.path.dirname(__file__), 'data/indexes')
@@ -444,7 +463,7 @@ class BuildIndexTests(TestCase):
             with open(filea_path, 'rb') as fh:
                 self.assertEqual(b'file a', fh.read())
 
-    @skipIf(not getattr(os, 'symlink', None), 'Requires symlink support')
+    @skipIf(not can_symlink(), 'Requires symlink support')
     def test_symlink(self):
         repo_dir = tempfile.mkdtemp()
         self.addCleanup(shutil.rmtree, repo_dir)