Don't run Rubocop in parallel mode when running in stdin mode

Fixes #536. Fixes #530

Since stdin mode only operates on one file, there's no benefit trying to run in parallel mode anyway.

lib/standard/runners/rubocop.rb

@@ -6,14 +6,27 @@ class Rubocop
       def call(config)
         rubocop_runner = RuboCop::CLI::Command::ExecuteRunner.new(
           RuboCop::CLI::Environment.new(
-            config.rubocop_options,
+            without_parallelizing_in_stdin_mode(config.rubocop_options),
             config.rubocop_config_store,
             config.paths
           )
         )
 
         rubocop_runner.run
       end
+
+      private
+
+      # This is a workaround for an issue with how `parallel` and `stdin`
+      # interact when invoked in this way. See:
+      #   https://github.com/testdouble/standard/issues/536
+      def without_parallelizing_in_stdin_mode(options)
+        if options[:stdin]
+          options.delete(:parallel)
+        end
+
+        options
+      end
     end
   end
 end

test/standard/runners/rubocop_test.rb

@@ -75,6 +75,7 @@ def test_print_corrected_output_on_stdin
   def test_print_corrected_output_on_stdin_with_corrections_on_stderr
     fake_out, fake_err = do_with_fake_io do
       @subject.call(create_config(
+        parallel: true, # should be removed dynamically by us to prevent RuboCop from breaking
         autocorrect: true,
         stderr: true,
         stdin: "def Foo;'hi'end\n"