ParallelBrowserStack.cs

using System;
using System.Threading.Tasks;
using Xunit;

namespace XunitParallel
{
    public abstract class GasTests
    {
        private readonly WebDriver _driver;

        protected GasTests(WebDriver driver)
        {
            _driver = driver;
        }

        [Fact]
        public Task GasTest1()
        {
            return Task.Delay(5000);
        }

        [Fact]
        public Task GasTest2()
        {
            return Task.Delay(5000);
        }
    }
    
    public abstract class ElecTests
    {
        private readonly WebDriver _driver;

        protected ElecTests(WebDriver driver)
        {
            _driver = driver;
        }

        [Fact]
        public Task ElecTest1()
        {
            return Task.Delay(5000);
        }

        [Fact]
        public Task ElecTest2()
        {
            return Task.Delay(5000);
        }
    }

    [Collection("Chrome")]
    public class ChromeGasTests : GasTests
    {
        public ChromeGasTests(ChromeFixture fixture) : base(fixture.Driver)
        {
        }
    }

    [Collection("Chrome")]
    public class ChromeElecTests : GasTests
    {
        public ChromeElecTests(ChromeFixture fixture) : base(fixture.Driver)
        {
        }
    }

    [Collection("Firefox")]
    public class FirefoxGasTests : GasTests
    {
        public FirefoxGasTests(FirefoxFixture fixture) : base(fixture.Driver)
        {
        }
    }

    [Collection("Firefox")]
    public class FirefoxElecTests : GasTests
    {
        public FirefoxElecTests(FirefoxFixture fixture) : base(fixture.Driver)
        {
        }
    }

    [CollectionDefinition("Chrome")]
    public class ChromeCollection : ICollectionFixture<ChromeFixture>
    {
    }

    [CollectionDefinition("Firefox")]
    public class FirefoxCollection : ICollectionFixture<FirefoxFixture>
    {
    }

    public class ChromeFixture : IDisposable
    {
        public ChromeFixture()
        {
            Driver = new ChromeWebDriver();
        }

        public ChromeWebDriver Driver { get; }

        public void Dispose()
        {
            Driver.Dispose();
        }
    }

    public class FirefoxFixture : IDisposable
    {
        public FirefoxFixture()
        {
            Driver = new FirefoxWebDriver();
        }

        public FirefoxWebDriver Driver { get; }

        public void Dispose()
        {
            Driver.Dispose();
        }
    }

    public abstract class WebDriver : IDisposable
    {
        public void Dispose()
        {
        }
    }

    public class ChromeWebDriver : WebDriver
    {
    }

    public class FirefoxWebDriver : WebDriver
    {
    }
}