Add support for class and collection xunit fixtures

[delatrie]

Currently, there is no way to include a class- or collection-scoped fixture to a report produced by allure-xunit. They neither appear automatically nor could be included by [AllureBefore] or [AllureAfter] (applying the attributes causes the Value cannot be null error).

Expected behavior

The following code:

using Xunit;
using System;

public class ClassFixture : IDisposable
{
    public ClassFixture(){}

    public void Dispose(){}
}

public class CollectionFixture : IDisposable
{
    public CollectionFixture(){}

    public void Dispose(){}
}

[CollectionDefinition("My xunit collection")]
public class MyCollection : ICollectionFixture<CollectionFixture>{}

[Collection("My xunit collection")]
public class TestClass : IClassFixture<ClassFixture>
{
    public TestClass(ClassFixture _1, CollectionFixture _2){}

    [Fact]
    public void TestMehtod(){}
}

should produce the following report:

Motivation

High-scope fixtures are important test framework constructs and ideally should be supported by allure out-of-box for all frameworks.

Other considerations

1. There should be a way to exclude a fixture from the report
2. There should be a way to assign a custom name to a fixture (on a class or method levels)

See also #353.