Added a test for multisite preview page

[raduconst]

This change is meant to test the PR #357. The problem is that when you're previewing a page on a multisite using subdirectory (not subdomain), the link is

http://eflowmslocal.local/test1?page_id=7&preview_id=7&preview=true

instead of:

http://eflowmslocal.local/test1/?page_id=7&preview_id=7&preview=true

(it's missing the / between test1 and ?page_id).

The solution from the PR is the right one and I want to wrap this up into a test.

Basically, what I'm trying to do is:

1. check if the current installation is multisite;
2. create a new blog;
3. move to it;
4. create a new page;
5. check its preview link.

The problem is that the preview link seems to have a broken structure once I create the new blog. This is how the link looks:

http://httpexample.org/sample/?page_id=3&preview=true

The slash appears even though the preview link in the browser is still broken.

[raduconst]

I managed to test the call for the home_url function from fix_post_row_actions.

The second call of the home_url function from get_preview_link is made through a filter and I couldn't reach that point.
I could take advantage of the fact that the function is public but I don't know what are the parameters ($actions and $post) that I should use. I also thought about using a filter inside the test, like here, for example, https://core.trac.wordpress.org/browser/trunk/tests/phpunit/tests/multisite/network.php#L126
but I had no luck with that.

[raduconst]

Update: once the PR #442 will be merged, this PR will be enough for testing the slash issue on the preview link.