Investigate changes with Mac Os 10.14

The following list of test cases fail on this new version of Mac OS and don't on prior versiosn 10.12 and 10.13.

```
wcf\src\System.Private.ServiceModel\tests\Scenarios\Binding\Custom
- CustomBindingTests/DefaultSettings_Https_Text_Echo_RoundTrips_String
- CustomBindingTests/DefaultSettings_Tcp_Binary_Echo_RoundTrips_String
wcf\src\System.Private.ServiceModel\tests\Scenarios\Binding\Http
- Binding_Http_NetHttpsBindingTests/DefaultCtor_NetHttps_Echo_RoundTrips_String
- Binding_Http_NetHttpsBindingTests/NonDefaultCtor_NetHttps_Echo_RoundTrips_String
wcf\src\System.Private.ServiceModel\tests\Scenarios\Client\ChannelLayer
- DuplexChannelShapeTests/IDuplexSessionChannel_Https_NetHttpsBinding
- RequestReplyChannelShapeTests/IRequestChannel_Https_NetHttpsBinding
wcf\src\System.Private.ServiceModel\tests\Scenarios\Extensibility\WebSockets
- WebSocketTests/WebSocket_Https_Duplex_BinaryBuffered
- WebSocketTests/WebSocket_Https_Duplex_BinaryStreamed
- WebSocketTests/WebSocket_Https_Duplex_TextBuffered_KeepAlive
- WebSocketTests/WebSocket_Https_Duplex_TextStreamed
- WebSocketTests/WebSocket_Https_RequestReply_BinaryBuffered
- WebSocketTests/WebSocket_Https_RequestReply_TextBuffered_KeepAlive
wcf\src\System.Private.ServiceModel\tests\Scenarios\Security\TransportSecurity
- HttpsTests/CrossBinding_Soap11_EchoString
- HttpsTests/SameBinding_DefaultSettings_EchoString
- HttpsTests/SameBinding_Soap11_EchoString
- HttpsTests/SameBinding_Soap12_EchoString
- Https_ClientCredentialTypeTests/BasicAuthenticationInvalidPwd_throw_MessageSecurityException
- Https_ClientCredentialTypeTests/BasicAuthentication_RoundTrips_Echo
- IdentityTests/ServiceIdentityNotMatch_Throw_MessageSecurityException
- IdentityTests/VerifyServiceIdentityMatchDnsEndpointIdentity
-Tcp_ClientCredentialTypeCertificateCanonicalNameTests/Certificate_With_CanonicalName_Fqdn_Address_EchoString
-Tcp_ClientCredentialTypeTests/NetTcp_SecModeTrans_CertValMode_ChainTrust_Succeeds_ChainTrusted
-Tcp_ClientCredentialTypeTests/TcpClientCredentialType_None_With_ServerAltName_EchoString
```

With errors such as...
```
System.ServiceModel.Security.SecurityNegotiationException : Could not establish trust relationship for the SSL/TLS secure channel with authority 'wcfcoresrv5.cloudapp.net'.
---- System.Net.Http.HttpRequestException : The SSL connection could not be established, see inner exception.
-------- System.Security.Authentication.AuthenticationException : The remote certificate is invalid according to the validation procedure.
```

```
System.ServiceModel.Security.SecurityNegotiationException : The X.509 certificate CN=wcfcoresrv5.cloudapp.net, O=DO_NOT_TRUST, OU=Created by https://github.com/dotnet/wcf; 6701A1458DAF48BB8B0DBC1D5C379FCF90950C76 chain building failed. The certificate that was used has a trust chain that cannot be verified. Replace the certificate or change the certificateValidationMode. One or more certificates required to validate this certificate cannot be found.
---- System.IdentityModel.Tokens.SecurityTokenValidationException : The X.509 certificate CN=wcfcoresrv5.cloudapp.net, O=DO_NOT_TRUST, OU=Created by https://github.com/dotnet/wcf; 6701A1458DAF48BB8B0DBC1D5C379FCF90950C76 chain building failed. The certificate that was used has a trust chain that cannot be verified. Replace the certificate or change the certificateValidationMode. One or more certificates required to validate this certificate cannot be found.
```