Mock.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Plugin.Connectivity.Abstractions;

namespace UnitTestProject.Tests.Mock
{
    public class MockConnectivity : IConnectivity
    {
        public bool IsConnected => true;

        public IEnumerable<ConnectionType> ConnectionTypes => new[] { ConnectionType.WiFi };

        public IEnumerable<ulong> Bandwidths => new[] { (ulong)4000 };

        public event ConnectivityChangedEventHandler ConnectivityChanged;
        public event ConnectivityTypeChangedEventHandler ConnectivityTypeChanged;

        public void Dispose()
        {
           
        }

        public Task<bool> IsReachable(string host, int msTimeout = 5000)
        {
            return Task.FromResult(true);
        }

        public Task<bool> IsRemoteReachable(string host, int port = 80, int msTimeout = 5000)
        {
            return Task.FromResult(true);
        }
    }
}

Tests.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using UnitTestingPlugins;
using UnitTestingPlugins.Helpers;
using UnitTestProject.Tests.Mock;

namespace UnitTestProject.Tests
{
    [TestClass]
    public class UnitTest1
    {
        [TestInitialize]
        public void Setup()
        {
            Connectivity.Instance = new MockConnectivity();
        }

        [TestMethod]
        public void TestMethod1()
        {
            var myClass = new MyClass();
            Assert.AreEqual(5, myClass.Math(2, 3));
        }
    }
}