Add spec base (Not finished)

spec/lib/gitlab/background_migration/migrate_legacy_artifacts_spec.rb

+require 'spec_helper'
+
+describe Gitlab::BackgroundMigration::MigrateLegacyArtifacts, :migration, schema: 20180427161409 do
+  let(:projects) { table(:projects) }
+  let(:pipelines) { table(:ci_pipelines) }
+  let(:jobs) { table(:ci_builds) }
+  let(:job_artifacts) { table(:ci_job_artifacts) }
+
+  context 'when legacy artifacts exist' do
+    before do
+      projects.create!(id: 123, name: 'gitlab', path: 'gitlab-ce')
+      pipelines.create!(id: 1, project_id: 123, ref: 'master', sha: 'adf43c3a')
+  
+      jobs.create!(id: 1, commit_id: 1, project_id: 123, status: :success)
+      jobs.create!(id: 2, commit_id: 1, project_id: 123, status: :success)
+      jobs.create!(id: 3, commit_id: 1, project_id: 123, status: :failed)
+      jobs.create!(id: 4, commit_id: 1, project_id: 123, status: :success)
+      jobs.create!(id: 5, commit_id: 1, project_id: 123, status: :pending)
+      jobs.create!(id: 6, commit_id: 1, project_id: 123, status: :pending)
+    end
+
+    it 'migrates' do
+
+      # And file access
+    end
+
+    context 'when job_artifacts has been already existed' do
+      it 'migrates' do
+
+        # And file access
+      end
+    end
+  end
+
+  context 'when legacy artifacts do not exist' do
+
+  end
+
+  def create_file(job, object_storage = false)
+    Tmpfile.create
+    JobArtifactUploader.
+    job.update_column(artifacts_file: nil)
+    job.update_column(artifacts_file_store: nil)
+    job.update_column(artifacts_size: nil)
+    job.update_column(artifacts_metadata: nil)
+    job.update_column(artifacts_metadata_store: nil)
+  end
+end