Add a describe_on_supported_os DSL

It is quite tedious to write the on_supported_os loop in every file.
This defines a short hand in the RSpec DSL. To keep access to the OS
facts, it is added to the metadata.

README.md

@@ -185,6 +185,23 @@ describe 'myclass' do
 end
 ```
 
+```ruby
+require 'spec_helper'
+
+describe_on_supported_os 'myclass' do
+  it { is_expected.to compile.with_all_deps }
+  ...
+
+  # If you need any to specify any operating system specific tests
+  case metadata[:os_facts][:osfamily]
+  when 'Debian'
+    ...
+  else
+    ...
+  end
+end
+```
+
 When using roles and profiles to manage a heterogeneous IT estate, you can test a profile that supports several OSes with many `let(:facts)` as long as each is in its own context:
 ```ruby
 require 'spec_helper'

lib/rspec-puppet-facts.rb

@@ -390,8 +390,26 @@ def self.facter_version_for_puppet_version(puppet_version)
   ensure
     fd.close if fd
   end
+
+  module DSL
+    def describe_on_supported_os(*args, &block)
+      describe(*args) do
+        # TODO: assumes RspecPuppetFacts was included
+        on_supported_os.each do |os, os_facts|
+          context("on #{os}") do
+            let(:facts) { os_facts }
+
+            class_exec(&block)
+          end
+        end
+      end
+    end
+  end
 end
 
+RSpec::Core::ExampleGroup.extend(RspecPuppetFacts::DSL)
+RSpec::Core::DSL.expose_example_group_alias(:describe_on_supported_os)
+
 RSpec.configure do |c|
   c.add_setting :default_facter_version,
     :default => RspecPuppetFacts.facter_version_for_puppet_version(Puppet.version)