Automatically report SetUps and TearDowns of NUnit's test fixtures

[delatrie]

Currently, to make test-level fixtures appears in the report, one has to mark them with [AllureBefore] (for SetUp methods) or [AllureAfter] (for TearDown methods).

We should pick the fixtures up automatically. In other words, this:

using NUnit.Framework;

public class TestClass
{
    [SetUp]
    public void SetUp() {}

    [Test]
    public void TestMethod(){}

    [TearDown]
    public void TearDown() {}
}

should produce the same report as this:

using NUnit.Allure.Attributes;
using NUnit.Framework;

public class TestClass
{
    [SetUp]
    [AllureBefore]
    public void SetUp() {}

    [Test]
    public void TestMethod(){}

    [TearDown]
    [AllureAfter]
    public void TearDown() {}
}

Motivation

1. Improve the out-of-box experience.
2. Put allure-nunit more in line with other allure integrations.

Considerations

The following should be supported to not take away features, users already have:

1. Specify a custom name of a fixture (via a new single attribute that replaces [AllureBefore] and [AllureAfter], a special attribute like [AllureTitle] or [AllureName] or, probably, a built-in [DisplayName]).
2. Provide an ability to exclude a fixture from the report for those who prefer to hide some technical fixtures (again, via a new abovementioned attribute or something like [AllureIgnore] or maybe other means).

Backward compatibility

[AllureBefore] and [AllureAfter] should still be supported (although, marked as obsolete).

See also

allure-framework/allure-csharp#359