Allure-pytest: report.longrepr information just available in allure web app if test fails with an exception

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#### I'm submitting a ...
- [ ] bug report
- [X] feature request
- [ ] support request => Please do not submit support request here, see note at the top of this template.

#### What is the current behavior?

Allure-pytest plugin just show failure information if the test throws an exception.

https://github.com/allure-framework/allure-python/blob/5e7bc860b5ddc1e072b0302022669183f2c4321f/allure-pytest/src/listener.py#L161-L182

#### What is the motivation / use case for changing the behavior?
Some useful plugins, like [pytest-check](https://github.com/okken/pytest-check), allow a non-fatal assertion behaviour modifying "report.longrepr" variable. "report.longrepr" could be valuable to get extra information in case of fail without exception information.

#### Please tell us about your environment:

- Allure version: 2.13.8
- Test framework: pytest@6.2.2
- Allure adaptor: allure-pytest@2.8.32

从 allure-pytest/src/listener.py 的第 161-182 行开始，检查当前如何收集失败信息。跟踪 pytest 对无异常失败的报告处理流程，然后让 report.longrepr 在 Allure Web 报告中可用；当显示 pytest-check 等插件产生的非致命断言详细信息时，即表示完成。

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