Что такое xUnit, NUnit, MSTest — основные отличия?

public class PriceCalculatorTests
{
    private readonly PriceCalculator _sut = new();      // новый экземпляр на каждый тест — свежее состояние

    [Fact]
    public void Returns_zero_for_empty_cart() => Assert.Equal(0m, _sut.Total([]));

    [Theory]
    [InlineData(100, 1, 100)]
    [InlineData(100, 3, 300)]
    [InlineData(50, 10, 450)]            // скидка 10% от 10 штук
    public void Calculates_total(decimal price, int qty, decimal expected) =>
        Assert.Equal(expected, _sut.Total([new CartLine(price, qty)]));
}

[TestFixture]
public class PriceCalculatorTests
{
    private PriceCalculator _sut = null!;

    [SetUp] public void SetUp() => _sut = new PriceCalculator();

    [Test]
    public void Returns_zero_for_empty_cart() => Assert.That(_sut.Total([]), Is.EqualTo(0m));

    [TestCase(100, 1, 100)]
    [TestCase(100, 3, 300)]
    [TestCase(50, 10, 450)]
    public void Calculates_total(decimal price, int qty, decimal expected) =>
        Assert.That(_sut.Total([new CartLine(price, qty)]), Is.EqualTo(expected));
}

[TestClass]
public class PriceCalculatorTests
{
    private PriceCalculator _sut = null!;

    [TestInitialize] public void Init() => _sut = new PriceCalculator();

    [TestMethod]
    public void Returns_zero_for_empty_cart() => Assert.AreEqual(0m, _sut.Total([]));

    [DataTestMethod]
    [DataRow(100, 1, 100)]
    [DataRow(100, 3, 300)]
    [DataRow(50, 10, 450)]
    public void Calculates_total(int price, int qty, int expected) =>
        Assert.AreEqual((decimal)expected, _sut.Total([new CartLine(price, qty)]));
}

public sealed class PostgresFixture : IAsyncLifetime
{
    private readonly PostgreSqlContainer _pg = new PostgreSqlBuilder().WithImage("postgres:17").Build();
    public string ConnectionString => _pg.GetConnectionString();
    public Task InitializeAsync() => _pg.StartAsync();
    public Task DisposeAsync() => _pg.DisposeAsync().AsTask();
}

[CollectionDefinition("db")]
public sealed class DbCollection : ICollectionFixture<PostgresFixture>;   // общий на несколько классов

[Collection("db")]
public class OrderRepositoryTests(PostgresFixture db, ITestOutputHelper output)
{
    [Fact]
    public async Task Saves_order() { /* использует db.ConnectionString */ }
}

// Встроенные
Assert.Equal(expected, actual);
Assert.Throws<DomainException>(() => order.Pay());
await Assert.ThrowsAsync<NotFoundException>(() => svc.GetAsync(id));

// NUnit constraint model
Assert.That(list, Has.Count.EqualTo(3).And.All.Property("IsActive").True);

// Shouldly / FluentAssertions — читаемые сообщения об ошибках
order.Total.ShouldBe(450m);
result.Should().BeEquivalentTo(expectedDto, o => o.Excluding(x => x.CreatedAt));