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.