Wrap tests around Jekyll::RelatedPosts

This gives me more confidence that we're doing the right things when it
comes to both the LSI and non-LSI cases and prevents regressions.

test/test_related_posts.rb

@@ -0,0 +1,40 @@
+require 'helper'
+
+class TestRelatedPosts < Test::Unit::TestCase
+  context "building related posts without lsi" do
+    setup do
+      stub(Jekyll).configuration do
+        Jekyll::Configuration::DEFAULTS.merge({'source' => source_dir,
+                                               'destination' => dest_dir})
+      end
+      @site = Site.new(Jekyll.configuration)
+    end
+
+    should "use the most recent posts for related posts" do
+      @site.reset
+      @site.read
+      assert_equal @site.posts[0..9], Jekyll::RelatedPosts.new(@site.posts.last).build
+    end
+  end
+
+  context "building related posts with lsi" do
+    setup do
+      stub(Jekyll).configuration do
+        Jekyll::Configuration::DEFAULTS.merge({'source' => source_dir,
+                                               'destination' => dest_dir,
+                                               'lsi' => true})
+      end
+      @site = Site.new(Jekyll.configuration)
+    end
+
+    should "use lsi for the related posts" do
+      @site.reset
+      @site.read
+      require 'classifier'
+      any_instance_of(::Classifier::LSI) do |c|
+        stub(c).find_related { @site.posts[-1..-9] }
+      end
+      assert_equal @site.posts[-1..-9], Jekyll::RelatedPosts.new(@site.posts.last).build
+    end
+  end
+end