Add specs for null reporter

spec/unit/mutant/reporter/null_spec.rb

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Mutant::Reporter::Null do
+  let(:object) { described_class.new }
+
+  describe '#report' do
+    subject { object.report(double('some input')) }
+
+    it_should_behave_like 'a command method'
+  end
+end