Hi Poornima. Based on your example code (and our DM conversation on LinkedIn) it created an example of how a test for your example could look like.

The base class UmbracoBaseTest is the same base class I mention on this page: https://our.umbraco.com/documentation/Implementation/Unit-Testing/

public class TestingTests : UmbracoBaseTest
    {
        private readonly UmbracoHelper umbracoHelper;
        private readonly Mock<IPublishedContent> currentPage;
        private readonly RegistrationController controller;

        public TestingTests()
        {
            base.SetUp();
            this.currentPage = new Mock<IPublishedContent>();
            this.umbracoHelper = new UmbracoHelper(this.currentPage.Object, Mock.Of<ITagQuery>(), this.CultureDictionaryFactory.Object, Mock.Of<IUmbracoComponentRenderer>(), this.PublishedContentQuery.Object, this.MembershipHelperImplementation);
            this.controller = new RegistrationController(Mock.Of<IUmbracoContextAccessor>(), Mock.Of<IUmbracoDatabaseFactory>(), base.ServiceContext, AppCaches.NoCache, Mock.Of<ILogger>(), Mock.Of<IProfilingLogger>(), umbracoHelper);
        }

        [Fact]
        public void Given_SuccessfullRegistration_When_PostRegistrationForm_Then_RedirectToThankYouPageWithExpectedQueryString()
        {
            var thankYouPageType = new Mock<IPublishedContentType>();
            thankYouPageType.Setup(x => x.Alias).Returns("thankYouPage");

            var thankYouPage = new Mock<IPublishedContent>();
            thankYouPage.Setup(x => x.Id).Returns(123);
            thankYouPage.Setup(x => x.Url).Returns("/thankyou");
            thankYouPage.Setup(x => x.ContentType).Returns(thankYouPageType.Object);

            this.currentPage.Setup(x => x.ChildrenForAllCultures).Returns(new List<IPublishedContent>() { thankYouPage.Object });

            var registrationModel = new ChosenRegistrationViewModel()
            {
                Id = 456,
                SignUpEventId = 789
            };

            var result = (RedirectResult)this.controller.PostRegistrationForm(registrationModel);

            result.Url.ShouldBe("/thankyou?eventId=789&registrationId=456");
        }
    }

    public class RegistrationController : SurfaceController
    {
        private readonly UmbracoHelper umbracoHelper;

        public RegistrationController(IUmbracoContextAccessor umbracoContextAccessor,
            IUmbracoDatabaseFactory umbracoDatabaseFactory, ServiceContext serviceContext, AppCaches appCaches,
            ILogger logger, IProfilingLogger profilingLogger, UmbracoHelper umbracoHelper) : base(
            umbracoContextAccessor, umbracoDatabaseFactory, serviceContext, appCaches, logger, profilingLogger,
            umbracoHelper)
        {
            this.umbracoHelper = umbracoHelper;
        }

        [HttpPost]
        public ActionResult PostRegistrationForm(ChosenRegistrationViewModel model)
        {
            //do some logic and if successful redirect

            var thankYouPage = this.umbracoHelper.AssignedContentItem.ChildrenOfType("thankYouPage").FirstOrDefault();
            return Redirect(string.Format($"{thankYouPage?.Url}?eventId={model.SignUpEventId}&registrationId={model.Id}"));
        }
    }

    public class ChosenRegistrationViewModel
    {
        public int Id { get; set; }
        public int SignUpEventId { get; set; }
    }

I ended up ditching RedirectToUmbracoPageResult because it turned out to be quite hard to Assert (queryStringValues is a private field, Url fields uses static resolver Current to resolve current page instead of DI), instead I used plain old return Redirect(url); Hope this works for you.

Have a great day and stay safe!