Testcases: remove "latest" if it is a symbolic link

Long description:

Test for the existence of the symlink to "latest" with the -l flag,
which tests if the target is a symbolic link. Testing with the -e flag
will fail in case the link points to a file that does not exist, which
will occur if the test result directories are deleted by hand.

To apply this patch, use:
curl http://cr.i3wm.org/patch/267/raw.patch | git am

@@ -108,7 +108,7 @@ $outdir .= POSIX::strftime("%Y-%m-%d-%H-%M-%S-", localtime());

 $outdir .= `git describe --tags`;

 chomp($outdir);

 mkdir($outdir) or die "Could not create $outdir";

-unlink("latest") if -e "latest";

+unlink("latest") if -l "latest";

 symlink("$outdir", "latest") or die "Could not symlink latest to $outdir";