gh-116858: Add @cpython_only to several tests in test_cmd_line

[sobolevn]

List of tests and reasoning:

• test_help, test_help_env, test_help_xoptions, test_help_all make assumptions about the exact output of help, which might not be the case for other implementations. They can show help as tables / with -X prefix, or whatever
• test_version explicitly tests that it starts with Python (it might be RustPython for other implementaions)
• test_env_var_frozen_modules and test_xoption_frozen_modules uses frozenmodules, which a basically our internal implementation detail that uses C parts
• test_unknown_options expects a very specific error message
• test_xdev uses malloc checks inside

CC @youknowone from RustPython, it might be interesting to you.

• Issue: Mark some tests test_cmd_line as cpython_only #116858

[youknowone]

Thank you! This is helpful!

[Eclips4]

I have some experience in "porting" tests from CPython to RustPython and this PR seems great! Thanks, Nikita.

[Eclips4]

Also, I would like to backport these changes to 3.11 & 3.12 branches. What do you think about this, Nikita?

[miss-islington-app]

Thanks @sobolevn for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @sobolevn, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker a1c4923d65a3e4cea917745e7f6bc2e377cde5c5 3.12

[miss-islington-app]

Sorry, @sobolevn, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker a1c4923d65a3e4cea917745e7f6bc2e377cde5c5 3.11

[sobolevn]

I will do the backports manually! 👍

[bedevere-app]

GH-116889 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-116890 is a backport of this pull request to the 3.11 branch.