Cover allure-xunit and allure-nunit with tests

[delatrie]

Currently we have some examples of allure-xunit and allure-nunit that could be run for manual check of the produced report. Examples are great for end users but not enough to fight regressions and be sure we properly cover all functionality.

What is the expected behavior?

We have tests on the Allure.Net.Commons and Allure.SpecflowPlugin package. We need also to cover the code in Allure.XUnit and Allure.NUnit with unit and acceptance tests as well.

Motivation

1. Prevent regressions.
2. Express implementation decisions.
3. Express how test framework constructs are supported by allure.
4. Express allure API available to users.

Other information

It is possible that this issue will be further decomposed into smaller ones.

Tasks

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1. Decide on what test framework to utilize
2. Implement auxilary code (test framework runners, allure abstractions, allure test objects) for acceptance testing
3. Cover the code with tests
4. Enhance the build pipeline to test against multiple runtimes