Implement fact memoization

Subsequent calls to on_supported_os with the same options should result
in the same facts. This can greatly speed up testing by reducing calls
to FacterDB.

lib/rspec-puppet-facts.rb

@@ -7,6 +7,8 @@
 # module's RSpec tests by looping through all supported
 # OS'es and their facts data which is received from the FacterDB.
 module RspecPuppetFacts
+  FACTS_CACHE = {}
+
   # Use the provided options or the data from the metadata.json file
   # to find a set of matching facts in the FacterDB.
   # OS names and facts can be used in the Puppet RSpec tests
@@ -27,6 +29,20 @@ module RspecPuppetFacts
   # will be used instead of the "operatingsystem_support" section if the metadata file
   # even if the file is missing.
   def on_supported_os(opts = {})
+    result = FACTS_CACHE[opts.to_s] ||= on_supported_os_implementation(opts)
+
+    # Marshalling is used to get unique instances which is needed for test
+    # isolation when facts are overridden.
+    Marshal.load(Marshal.dump(result))
+  end
+
+  # The real implementation of on_supported_os.
+  #
+  # Generating facts is slow - this allows memoization of the facts between
+  # multiple calls.
+  #
+  # @api private
+  def on_supported_os_implementation(opts = {})
     opts[:hardwaremodels] ||= ['x86_64']
     opts[:hardwaremodels] = [opts[:hardwaremodels]] unless opts[:hardwaremodels].is_a? Array
     opts[:supported_os] ||= RspecPuppetFacts.meta_supported_os