Add empty test coverage badge class and specs

lib/gitlab/badge/coverage.rb

+module Gitlab
+  module Badge
+    ##
+    # Test coverage badge
+    #
+    class Coverage
+      def initialize(project, ref, job = nil)
+        @project = project
+        @ref = ref
+        @job = job
+      end
+
+      def coverage
+      end
+    end
+  end
+end

spec/lib/gitlab/badge/coverage_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Badge::Coverage do
+  let(:project) { create(:project) }
+
+  let(:pipeline) do
+    create(:ci_pipeline, project: project,
+                         sha: project.commit.id,
+                         ref: 'master')
+  end
+
+  let(:badge) { described_class.new(project, 'master') }
+
+  context 'builds exist' do
+  end
+
+  context 'build does not exist' do
+  end
+end