using HWSysManagerLib;
bool ProcessBias(HWSysManager systemManager, string hwPath)
{
    int handle = systemManager.OpenConfiguration(hwPath);
    ...
    // some magic goes here
    // return result
}

public interface IHWSysManager
{
    int OpenConfiguration(string hwPath);
}

public class HWSysManagerImpl : IHWSysManager
{
    private HWSysManager _hwSysManager; //Initialize from constructor

    public int OpenConfiguration(string hwPath)
    {
        return _hwSysManager.openConfiguration(hwPath);
    }
}

bool ProcessBias(IHWSysManager systemManager, string hwPath)
{
    int handle = systemManager.OpenConfiguration(hwPath);
    ...
    // some magic goes here
    // return result
}

var fakeManager = Isolate.Fake.Instance<HWSysManager>();

Isolate.WhenCalled(() => fakeManager.OpenConfiguration("")).WillReturn(0);

Isolate.WhenCalled(() => fakeManager.OpenConfiguration("")).DoInstead(
    context =>
    {
        //Your simulation
    });

class HWSysManager
    {
        public virtual int ExampleReturnIntMethod(int a)
        {
            var someInt = 0;
            return someInt;
        }

public void TestMethod()
    {
        Mock<HWSysManager> hwSysManager = new Mock<HWSysManager>();
        hwSysManager.Setup(x => x.ExampleReturnInMethod(It.IsAny<int> ())).Returns(10); //if parameter is any of int, return 10 in this case
    }

var hwSysInstance = hwSysManager.Object;
 var result = hwSysInstance.ExampleReturnInMethod(2); //result will be 10 in this case - as we have mocked

public interface HwsysManager
    {
        int OpenConfiguration(string hwPath);
    }

     public void TestMethod()
    {
      Mock<HwsysManager> hwsysManager = new Mock<HwsysManager>();

      hwsysManager.Setup(x => x.OpenConfiguration(It.IsAny<string>())).Returns(10);
    }