Ruby: Add specs for KernAux::File

pkgs/ruby/lib/kernaux/file.rb

@@ -2,6 +2,7 @@
 
 # rubocop:disable Lint/EmptyClass
 
+if defined? KernAux::File
   module KernAux
     ##
     # File simulator.
@@ -16,5 +17,6 @@ module KernAux
       # @!parse [ruby]
     end
   end
+end
 
 # rubocop:enable Lint/EmptyClass

pkgs/ruby/spec/lib/kernaux/file_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+defined? KernAux::File and RSpec.describe KernAux::File do
+  subject(:our_file) { described_class.new method :out }
+
+  def out(str)
+    (@buffer ||= +'') << String(str)
+  end
+
+  specify do
+    expect(our_file.instance_variable_get(:@out)).to eq method :out
+  end
+end