Add support for NUnit's SetUpFixtures

[delatrie]

Currently, SetUpFixtures are not included into the report. Ideally, they should be collected automatically, with no additional efforts, so the following code:

using NUnit.Framework;
using NUnit.Allure.Core;

[SetUpFixture]
public class MyFixture
{
    [OneTimeSetUp]
    public void SetUp(){}

    [OneTimeTearDown]
    public void TearDown(){}
}

public class TestClass
{
    [Test]
    public void TestMethod(){}
}

should produce the following report:

Motivation

Same as #360: fixtures should be supported out-of-box.

Other information

Again, as for #360:

1. There should be a way to preclude a fixture from getting in the report for those who are used to the current opt-in approach to fixtures.
2. There should be a way to assign a custom name to a fixture (on a class or method levels).