Add spec for parsers factory

spec/lib/gitlab/ci/parsers_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Parsers do
+  describe '.fabricate!' do
+    subject { described_class.fabricate!(file_type) }
+
+    context 'when file_type exists' do
+      let(:file_type) { 'junit' }
+
+      it 'fabricates the class' do
+        is_expected.to be_a(described_class::Junit)
+      end
+    end
+
+    context 'when file_type does not exist' do
+      let(:file_type) { 'undefined' }
+
+      it 'raises an error' do
+        expect { subject }.to raise_error(NameError)
+      end
+    end
+  end
+end