feat: pytest marks to selectively skip some tests

[dougch]

Resolved issues:

Partial for https://github.com/aws/s2n-tls/issues/3841

Description of changes:

Adds a pytest mark to allow skipping tests based on current platform (x86 or arm) and if we detect a nix devshell.

Marks added to skip certain tests based on local testing.

Call-outs:

The mark uncollect_if() was moved from conftest.py to the pytest.ini.

While pytest.ini is normally at the root of the project, we've historically passed all of the arguments into pytest instead of using a configuration file.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? Locally.

Is this a refactor change? No.

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