Disable invalid tests on Windows.

Requires jekyll/jekyll#2109.

Mainly this just tweaks the tests to ignore symlinks. Other than this, all the tests pass on Windows currently with the other PR merged as well. The only tests that are failing for me are as follows and I would put these down to issues with timezones; if so, they're tangential to this PR:

```
[160/330] TestPost#test: A Post initializing posts rendering should render time specified in front matter prope
  1) Failure:
test: A Post initializing posts rendering should render time specified in front matter properly. (TestPost) [D:/Programming/jekyll/test/test_post.rb:560]:
<"<p>Post with a front matter time</p>\n<p>10 Jan 2010</p>"> expected but was
<"<p>Post with a front matter time</p>\n<p>11 Jan 2010</p>">.

[177/330] TestPost#test: A Post initializing posts should recognize time in yaml.  = 0.00 s
  2) Failure:
test: A Post initializing posts should recognize time in yaml. (TestPost) [D:/Programming/jekyll/test/test_post.rb:423]:

<"/2010/01/10/time-override.html"> expected but was
<"/2010/01/11/time-override.html">.

[178/330] TestPost#test: A Post initializing posts should recognize time with timezone in yaml.  = 0.00 s
  3) Failure:
test: A Post initializing posts should recognize time with timezone in yaml. (TestPost) [D:/Programming/jekyll/test/test_post.rb:432]:
<"/2010/01/10/timezone-override.html"> expected but was
<"/2010/01/11/timezone-override.html">.
```