Turn on xfail_strict = true for all python packages

[wence-]

Description

The cudf tests already treat tests that are expected to fail but pass as errors, but at the time we introduced that change, we didn't do the same for the other packages. Do that now, it turns out there are only a few xpassing tests.

While here, it turns out that having multiple different pytest configuration files does not work. pytest.ini takes precedence over other options, and it's "first file wins". Consequently, the merge of #16851 turned off xfail_strict = true (and other options) for many of the subpackages.

To fix this, migrate all pytest configuration into the appropriate section of the pyproject.toml files, so that all tool configuration lives in the same place. We also add a section in the developer guide to document this choice.

• Closes Turn on xfail_strict=true in all subpackages #12391
• Closes [BUG] some pytest configurations not used #16974

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[jameslamb]

Totally support consolidating all these settings in pyproject.toml... that mix of pytest.ini and pyproject.toml here has tripped me up in the past too.

[mroeschke]

Some optional options to set in a follow-up if interested to make pytest more strict

• empty_parameter_set_mark = "fail_at_collect" : https://docs.pytest.org/en/stable/reference/reference.html#confval-empty_parameter_set_mark
• addopts = "--strict-config --strict-markers"

[wence-]

• --strict-config --strict-markers

I added these, let's see

[wence-]

/merge