Implement Cucumber feature showing built-in formatters configuration.

features/config_formatters.feature

@@ -0,0 +1,52 @@
+@test_unit @config
+Feature:
+
+  The formatter for test coverage can be customized
+  with the SimpleCov.formatter setting. There are two
+  built-in formatters:
+    SimpleCov::Formatter::SimpleFormatter is a simple
+    formatter returning a string of all files with
+    theirs coverages.
+    SimpleCov::Formatter::MultiFormatter is a formatter
+    used to call multiple formatters at once.
+
+  Scenario: With SimpleFormatter
+    Given SimpleCov for Test/Unit is configured with:
+      """
+      require 'simplecov'
+      SimpleCov.formatter = SimpleCov::Formatter::SimpleFormatter
+      SimpleCov.at_exit do
+        puts SimpleCov.result.format!
+      end
+      SimpleCov.start do
+        add_group 'Libs', 'lib/faked_project/'
+      end
+      """
+
+    When I successfully run `bundle exec rake test`
+    Then the output should contain "lib/faked_project/meta_magic.rb (coverage: 100.0%)"
+
+  Scenario: With MultiFormatter
+    Given SimpleCov for Test/Unit is configured with:
+      """
+      require 'simplecov'
+      SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter[
+        SimpleCov::Formatter::SimpleFormatter,
+        Class.new do
+          def format(result)
+            raise "Unable to format"
+          end
+        end
+      ]
+
+      SimpleCov.at_exit do
+        puts SimpleCov.result.format!.join
+      end
+      SimpleCov.start do
+        add_group 'Libs', 'lib/faked_project/'
+      end
+      """
+
+    When I successfully run `bundle exec rake test`
+    Then the output should contain "lib/faked_project/meta_magic.rb (coverage: 100.0%)"
+    And the output should match /Formatter [^\s]* failed with RuntimeError: Unable to format/