Drop private imports on test_utils.py

[T-256]

Summary

closes #2492

get_ca_bundle_from_env moved to where it's calling.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[karpetrosyan]

Please optimize the number of git commits.
You can even do fewer commits or force push them at the end so that we only see a few commits rather than the commit army.
I believe it will make the pull request more attractive and easier to review.

[tomchristie]

Okay, so the intention here is to change test_utils.py so that we're only ever testing against public API.

There's a mix here of some tests that do that, and some other tests that correctly switch to only using public imports, but switch to using private accessors.

I could review the PR as a whole, but it's a bit of a mix.

Perhaps an easier approach to getting this resolved would be to only target one type of change at a time.

[T-256]

but switch to using private accessors.

previous PR like this also used private accessor, so I thought it's no problem using them in tests at all.

[T-256]

close in favor of separating to chunks...