CertificateClientLiveTests should reset challenge cache in record and playback modes

[heaths]

Other client tests for Key Vault have code like the following we should add to CertificateClientLiveTests for the reasons stated in the comment:

        [SetUp]
        public void ClearChallengeCacheforRecord()
        {
            // in record mode we reset the challenge cache before each test so that the challenge call
            // is always made.  This allows tests to be replayed independently and in any order
            if (Mode == RecordedTestMode.Record || Mode == RecordedTestMode.Playback)
            {
                Client = GetClient();

                ChallengeBasedAuthenticationPolicy.AuthenticationChallenge.ClearCache();
            }
        }