it('the postcode lookup client returns an error', async () => {
        nock(mockPostcodeServer)
          .get(mockPostcodePath)
          .reply(500)

        expect(await constraint.validate('SW2 1AN', validationArgs(Country.all()))).to.be.true
      })

 it('should return undefined for unknown country', () => {
   chai.expect(Country.valueOf('unknown-country')).to.be.undefined
 })

 Country.all().forEach(country => {
   chai.expect(Country.valueOf(country.value)).to.be.equal(country)
 })