Implement rspec function it_raises

On some puppet modules, a it_raises helper is provided. This commit
implements it for puppet-swift.

Change-Id: Id8724823b912f201cc137e23f991d8e05acdcbcc

spec/shared_examples.rb

@@ -0,0 +1,5 @@
+shared_examples_for "a Puppet::Error" do |description|
+  it "with message matching #{description.inspect}" do
+    expect { should have_class_count(1) }.to raise_error(Puppet::Error, description)
+  end
+end

spec/spec_helper.rb

@@ -1,5 +1,7 @@
 require 'puppetlabs_spec_helper/module_spec_helper'
+require 'shared_examples'
 
 RSpec.configure do |c|
   c.alias_it_should_behave_like_to :it_configures, 'configures'
+  c.alias_it_should_behave_like_to :it_raises, 'raises'
 end