added cookstyle unit test

spec/overcommit/hook/pre_commit/cook_style_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Overcommit::Hook::PreCommit::CookStyle do
+  let(:config)  { Overcommit::ConfigurationLoader.default_configuration }
+  let(:context) { double('context') }
+  subject { described_class.new(config, context) }
+
+  before do
+    subject.stub(:applicable_files).and_return(%w[file1.rb file2.rb])
+  end
+
+  context 'when cookstyle exits successfully' do
+    let(:result) { double('result') }
+
+    before do
+      result.stub(success?: true, stderr: '', stdout: '')
+      subject.stub(:execute).and_return(result)
+    end
+
+    it { should pass }
+  end
+
+  context 'when cookstyle exits unsucessfully' do
+    let(:result) { double('result') }
+
+    before do
+      result.stub(:success?).and_return(false)
+      subject.stub(:execute).and_return(result)
+    end
+
+    it { should fail_hook }
+  end
+end