Add ability to capture failed test outut for Xunit

[AngryTester95]

Context

Currently, Allure.Xunit does not have the functionality to capture output for failed tests, as opposed to passed ones. But this is important, especially for failed tests.

Checklist

• [x] Sign Allure CLA
• [ ] Provide unit tests

[CLAassistant]

All committers have signed the CLA.