Add support for async NUnit tests

[overlord]

This pull request provides workaround or maybe even fixes the issues (in Allure.NUnit 2.9.2-preview.1) with async test methods in NUnit.Allure.

• Test doesn't wait when async step finishes #303
• If test contains await, then [AllureStep] failed with System.ArgumentNullException : Value cannot be null. (Parameter 'key') unickq/allure-nunit#28 (same issue in old lib)

It fixes the case when async methods marked with [AllureStep] like the following:

[AllureStep("...")]
public async Task EnsureDeletedStep(string itemCode)
{ ... }

fails in runtime with exception like the following:

System.ArgumentNullException : Value cannot be null. (Parameter 'key')
---
Stack Trace:
ThrowHelper.ThrowArgumentNullException(String name)
ConcurrentDictionary`2.TryGetValue(TKey key, TValue& value)
ConcurrentDictionary`2.get_Item(TKey key)
AllureStorage.Get[T](String uuid)
AllureStorage.AddStep(String parentUuid, String uuid, StepResult stepResult)
AllureLifecycle.StartStep(String parentUuid, String uuid, StepResult stepResult)
AllureLifecycle.StartStep(String uuid, StepResult result)
AllureStepAspect.WrapStep(String name, MethodBase methodBase, Object[] arguments, Func`2 method)
...

The main problem was in the ThreadStatic field AllureStorage.stepContext - that fails on cross-thread execution of async methods.

I have tried to fix it and have provided some unit tests for new async behaviour (see. Allure.NUnit.AsyncTests.AsyncLifeCycleTests)

[neparij]

Hello, @overlord !
Thank you for contributing to Allure!
I'll look your PR and provide the feedback soon.

Looks like it solves this issue completely.

[neparij]

Nice solution, could you look at review?

[neparij]

Remove accidentally added file please.

[neparij]

Needs to be removed