feature request: order tests by slowest-first

in long running test suites (especially with xdist), it's useful to run the slowest tests first so that you end up with better load. the following code seems to do so:
```
def pytest_collection_modifyitems(config, items):
try:
with open(".test_durations") as f:
s = f.read()
durations = json.loads(s)
except FileNotFoundError:
durations = {}

timings = {}
for item in items:
path, _, reqname = item.location
request_id = f"{path}::{reqname}"
timing = durations.get(request_id, None)
if timing is not None:
timings[item] = timing

meantime = sum(timings.values()) / len(timings)
# sort by highest time first
items.sort(key=lambda item: -timings.get(item, meantime))
```
but i was thinking it might be good for this to be available in the pytest-split plugin itself, so we don't need to add this same function across different projects.

alternatively, i was thinking publishing this as a plugin myself, if pytest-split does not want to add it.

首先阅读 pytest-split 在 collection 修改以及现有 .test_durations 处理方面的插件入口点。检查当前如何实现与 xdist 相关的拆分，以及所提议的 pytest_collection_modifyitems hook 是否适合该插件的流程。当插件能够在不要求重复编写项目特定代码的情况下，按最慢优先的顺序排列测试时，即视为完成。

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