Add test to preview path

spec/command/preview_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Apiary::Command::Preview do
+
+  it 'check tmp path if contains filename' do
+    opts = {}
+    command = Apiary::Command::Preview.new(opts)
+    expect(command.preview_path('apiary.apib')).to end_with('apiary.apib-preview.html')
+  end
+
+end