fix: move calls requiring integ test environment setup from test setup to inside test body

[mattsb42-aws]

Issue #, if available:

related to: pyca/cryptography#5197

Description of changes:

In a recent batch of test refactoring, I inadvertently moved some calls that relied on integ test environment setup (namely: setting certain environment variables) into the test setup that used to be within the test body. This caused these calls to be made during pytest test discovery. This is a problem because we use pytest markers to categorize our tests and test discovery must complete before markers can be applied to filter tests to run.

We have a test (TOXENV=nocmk) that should have caught this, but it looks like it was no longer removing the environment variable restrictions. That is fixed now, along with a comment explaining why that is important.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[alex]

flake8 appears unhappy

[mattsb42-aws]

flake8 appears unhappy

That's expected; in the process of moving our non-integ tests from Travis to GitHub, I enabled some checks on the test vector handler sub-module that were previously disabled. They're currently expected to fail (and so not required) but enabled to remind me to fix them.