remove collect_upstream_conda_tests and test_modified_upstream

[jaimergp]

Description

Now that conda/conda@main has merged 23.10 back in, we can rely directly on conda's unaltered test suite. To be decided if we want to keep doing this, but for now let's just use it as is.

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• Add / update outdated documentation?

[jaimergp]

_______________________ test_install_features[libmamba] ________________________
[XPASS(strict)] Features not supported in libmamba

Did we accidentally fix something in main? 😱 Noice if true! This one was passing (i.e. failing as expected) in conda/conda 🤔

[jaimergp]

Yep, I bisected through the recent history and that test started passing with 671b9c1 being merged. I'll remove the strict=True upstream.

[jaimergp]

Submitted conda/conda#13320 upstream. I'll merge if the reruns are successful. The test_install_feature will still xpass til that PR is merged.

[jaimergp]

tests/test_misc.py::test_explicit_missing_cache_entries fails on macOS because I assume the test order makes it have a PackageCache with an entry whose url is None. I added a patch in the same PR and will merge this one to unlock #365.