Add integration spec for 'null' strategy

spec/integration/mutant/null_spec.rb

@@ -0,0 +1,18 @@
+# encoding: utf-8
+
+require 'spec_helper'
+
+describe 'null integration' do
+
+  let(:base_cmd) { 'bundle exec mutant -I lib --require test_app "::TestApp*"' }
+
+  around do |example|
+    Dir.chdir(TestApp.root) do
+      example.run
+    end
+  end
+
+  specify 'it allows to kill mutations' do
+    expect(Kernel.system(base_cmd)).to be(true)
+  end
+end