Support NUnit's test classes out-of-box

[delatrie]

Currently, one has to apply the [AllureNUnit] attribute to all their NUnit test classes (test fixtures in NUnit's terminology) in order for them to appear in the report.

The goal is to make this step unnecessary, picking all test fixtures up automatically.

Motivation

It's a part of a larger goal to make allure-nunit usable completely out-of-box. See also #344, where a similar task related to allure-xunit is formulated.

Example

The following usage:

using NUnit.Framework;

namespace MyNamespace;

public class MyTestFixture
{
    [Test]
    public void MyTest(){}
}

should produce the same result, as one with the attribute:

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

namespace MyNamespace;

[AllureNUnit]
public class MyTestFixture
{
    [Test]
    public void MyTest(){}
}

Notes

Research is needed on whether it can be achieved and how. Currently, a framework extension mechanism is used to listen test execution events. We should take a closer look at engine extension mechanisms: Creating Engine Extensions. Chances are, one of them (or some combination of them) will suite out needs.

Backward compatibility

The [AllureNUnit] attribute should still be supported, although, obsoleted (it will become a noop basically).