A Fake Claims Identity for testing, with claims settable in code.

FakeClaimsIdentity.cs

using System.Collections.Generic;
using System.Security.Claims;
using System.Security.Principal;

public class FakeClaimsIdentity : ClaimsIdentity
{
    public readonly List<Claim> ClaimsValue = new List<Claim>();
    
    public bool IsAuthenticatedSet;

    public FakeClaimsIdentity() : base(new GenericIdentity(typeof(FakeClaimsIdentity).Name)) { }

    public FakeClaimsIdentity(string name) : base(new GenericIdentity(name)) { }

    public FakeClaimsIdentity(GenericIdentity identity) : base(identity) { }

    public override bool IsAuthenticated => IsAuthenticatedSet;

    public override IEnumerable<Claim> Claims => ClaimsValue;

    public FakeClaimsIdentity WithClaim(Claim claim)
    {
        AddClaim(claim);
        return this;
    }

    public override void AddClaim(Claim claim) { ClaimsValue.Add(claim); }

    public override void AddClaims(IEnumerable<Claim> claims) { ClaimsValue.AddRange(claims); }

    public override bool TryRemoveClaim(Claim claim) { return ClaimsValue.Remove(claim); }
}

Taken from https://www.nuget.org/packages/TestBase