Add specs for Gitlab::Plugin [ci skip]

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/lib/gitlab/plugin_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Plugin do
+  describe '.execute' do
+    let(:data) { Gitlab::DataBuilder::Push::SAMPLE_DATA }
+    let(:plugin) { Rails.root.join('plugins', 'test.rb') }
+    let(:tmp_file) { Tempfile.new('plugin-dump').path }
+
+    before do
+      File.write(plugin, plugin_source)
+      File.chmod(0o777, plugin)
+    end
+
+    after do
+      FileUtils.rm(plugin)
+      FileUtils.rm(tmp_file)
+    end
+
+    subject { described_class.execute(plugin.to_s, data) }
+
+    it { is_expected.to be true }
+
+    it 'ensures plugin received data via stdin' do
+      subject
+
+      expect(File.read(tmp_file)).to eq(data.to_json)
+    end
+  end
+
+  private
+
+  def plugin_source
+    <<-EOS
+#!/usr/bin/env ruby
+x = STDIN.read
+File.write('#{tmp_file}', x)
+    EOS
+  end
+end