test: fix flaky test for symlinks

If the symlink portion of the test was being skipped due to a
combination of OS support and user privileges, then an assertion would
always fail. This fixes that problem, improves assertion error reporting
and splits the test to make it clear that it is a test for links and
symlinks.

Fixes: https://github.com/nodejs/node/issues/3311
PR-URL: https://github.com/nodejs/node/pull/3418


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>