Reduce flakiness of CI test runs

[mbargull]

Description

I started to include unrelated test fixes in gh-4650 to easier inspect failing/passing of relevant tests.
Those started to grow and it's cleaner to have them separated, anyway, so here we go.

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[mbargull]

I didn't add a news entry because this is just chore that doesn't touch anything outside the test code.
If you'd still like to have a news entry, let me know.

[mbargull]

Re: config=testing_config:
Previously, tests failed because (even when only rendering) concurrent writes/deletions around the {croot}/work happened at conda_build.config.Config.compute_build_id (at the end in its if old_dir != work_dir: branch) happened. I didn't dig into it, but it might just be that another test creates the work (without a timestamp) folder and then compute_build_id wants to move it.
In any case, testing_config sets the croot to testing_workdir and as such the single tests use their own root dir without seeing others' work folders.

[jezdez]

We recently switched to pytest-rerunfailures because it's better maintained and used in the conda code base. You should be able to achieve the same as the rerun_filter parameter with the condition parameter: https://github.com/pytest-dev/pytest-rerunfailures#re-run-individual-failures

[mbargull]

We recently switched to pytest-rerunfailures because it's better maintained

Yeah, saw that too, but

You should be able to achieve the same as the rerun_filter parameter with the condition parameter: https://github.com/pytest-dev/pytest-rerunfailures#re-run-individual-failures

this appears to be just for the "outside" conditions but doesn't get the test's report information: https://github.com/pytest-dev/pytest-rerunfailures/blob/10.3/pytest_rerunfailures.py#L323-L324
I.e., pytest-rerunfailures seems to lack comparable/flexible functionality w.r.t. flaky.
AIUI, if we want to use pytest-rerunfailures, we have to either be non-selective at all or add --only-rerun=ConnectionError to addopts in setup.cfg and thus be too selective. Since the latter could become a footgun, the former is more sensible.

Do you want me to switch to pytest-rerunfailures and thus remove the selectivity on ConnectionError?

[mbargull]

Closing in favor of gh-4653