Provide default suites out-of-box for allure-nunit and allure-xunit

[delatrie]

Currently, no suite labels are added automatically either by allure-xunit or by allure-nunit.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

What is the expected behavior?

Add a parent suite if no parent suite label specified by a user.
Similarly, provide a suite if no suite label specified by a user.
Similarly, provide a sub-suite if no sub-suite label specified by a user.

The most convenient IMO are:

• Parent suite: an assembly name
• Suite: a namespace
• Sub-suite: a class name

This creates a four-level hierarchy of assemblies -> namespaces -> classes -> test cases, something quite familiar to .NET developers who use tools like ILSpy.

Motivation

Default suites provide reasonable and familiar navigation scheme through allure test cases that could be more convenient for some projects then package-based hierarchy (that has no depth limitations).