Client Certificate failures in CI machines on OSX10.12

[StephenBonikowsky]

After upgrading to netstandard2.0 we also had to update a number of infrastructure items. One of these required that we update the supported OSX to OSX10.12.

This change caused our certificate setup step to fail on CI machines so all test cases depending on these certificates now fail.

Identify and fix the setup script.

[hongdai]

List of test failures:
Non certificate related:
SessionTests.Test_IsInitiating_IsTerminating_Separate_Channels
SessionTests.Test_Implicit_Session_Initiation_And_Termination
SessionTests.Test_IsInitiating_IsTerminating

Service certificate or Client certificate related tests:
CustomBindingTests.DefaultSettings_Tcp_Binary_Echo_RoundTrips_String
IdentityTests.ServiceIdentityNotMatch_Throw_MessageSecurityException
IdentityTests.VerifyServiceIdentityMatchDnsEndpointIdentity
Tcp_ClientCredentialTypeCertificateCanonicalNameTests.Certificate_With_CanonicalName_DomainName_Address_EchoString
Tcp_ClientCredentialTypeCertificateCanonicalNameTests.Certificate_With_CanonicalName_Localhost_Address_EchoString
Tcp_ClientCredentialTypeCertificateCanonicalNameTests.Certificate_With_CanonicalName_Fqdn_Address_EchoString
Tcp_ClientCredentialTypeTests.NetTcp_SecModeTrans_CertValMode_PeerTrust_Succeeds_In_TrustedPeople
Tcp_ClientCredentialTypeTests.TcpClientCredentialType_Certificate_CustomValidator_EchoString
Tcp_ClientCredentialTypeTests.TcpClientCredentialType_Certificate_EchoString
Tcp_ClientCredentialTypeTests.NetTcp_SecModeTrans_Duplex_Callback_Succeeds
Tcp_ClientCredentialTypeTests.NetTcp_SecModeTrans_CertValMode_ChainTrust_Succeeds_ChainTrusted
Tcp_ClientCredentialTypeTests.NetTcp_SecModeTrans_CertValMode_PeerTrust_Fails_Not_In_TrustedPeople
Tcp_ClientCredentialTypeTests.TcpClientCredentialType_Certificate_With_ServerAltName_EchoString
Tcp_ClientCredentialTypeTests.NetTcp_SecModeTrans_CertValMode_PeerOrChainTrust_Succeeds_ChainTrusted
ExpectedExceptionTests.TCP_ServiceCertRevoked_Throw_SecurityNegotiationException
ExpectedExceptionTests.TCP_ServiceCertFailedCustomValidate_Throw_Exception
ExpectedExceptionTests.TCP_ServiceCertExpired_Throw_MessageSecurityException

Service certificate related tests are fixed by the fix to #1574

[hongdai]

Update: Client certificate tests passed on my local machine. This must be a CI set up issue. Will investigate further.

[hongdai]

This issue now will be ONLY used to track the Client Certificate failures in CI machine, caused by CI uses root account to run the tests. See https://github.com/dotnet/core-eng/issues/775

All other failures have been fixed/tracked by the following issues:
#1925
#1926