Add crono rake tasks spec

spec/tasks/crono_tasks_spec.rb

@@ -0,0 +1,14 @@
+require 'spec_helper'
+require 'rake'
+
+load 'tasks/crono_tasks.rake'
+Rake::Task.define_task(:environment)
+
+describe 'crono:clean' do
+  it 'should clean unused tasks from DB' do
+    Crono::CronoJob.create!(job_id: 'used_job')
+    expect(Crono).to receive(:load_cronotab)
+    Rake::Task['crono:clean'].invoke
+    expect(Crono::CronoJob.where(job_id: 'used_job')).not_to exist
+  end
+end