Add Negotiate and MultipleSchemes code to the System.net test Prerequisites

[CIPop]

Add the missing showidentity.ashx file required by DefaultCredentialsTest.cs

        // This test endpoint offers multiple schemes, Basic and NTLM, in that specific order. This endpoint
        // helps test that the client will use the stronger of the server proposed auth schemes and
        // not the first auth scheme.
        private static Uri MultipleSchemesAuthenticatedServer =
            new Uri($"http://{DomainJoinedTestServer}/test/auth/multipleschemes/showidentity.ashx");

[karelz]

This is about having multi-machine test support.

[davidsh]

For reference, the current scripts are here:
https://github.com/dotnet/corefx/tree/master/src/Common/tests/System/Net/Prerequisites

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