code: function MockCacheClient (sandbox) {
      return {
          get: sandbox.stub().resolves(null),
          set: sandbox.stub().resolves(),
          setex: sandbox.stub().resolves(),
      }
  }
docs: @function MockCacheClient
  
  create a new mock cache client with sinon sandbox
  
  @params {object} sandbox
  
  @returns {MockCacheClient}