test: make diagnostic helper name match behavior

[lewis6991]

Rename check_code_for to has_no_diagnostic so assertions describe the
existing behavior directly.

Assisted-by: Codex

[gemini-code-assist]

Code Review

This pull request inverts the boolean logic of the check_code_for function within the test library, resulting in a widespread update to assertions across the test suite to ensure tests continue to validate the presence or absence of diagnostics correctly. I have no feedback to provide.

[CppCXY]

If the logic is to be completely reversed, that usually means the current name is not appropriate. I would suggest renaming it rather than reversing the judgment logic.

[lewis6991]

A better name with the old logic would be check_no_code_for, but that just embeds the ! into the name. I do think keeping the name and reversing the result is better here. check_code_for means "run this code and check it raises this diagnostic".

[CppCXY]

A better name with the old logic would be , but that just embeds the into the name. I do think keeping the name and reversing the result is better here. means "run this code and check it raises this diagnostic".check_no_code_for``!``check_code_for

The problem with reversing the logic is that it would result in unmerged code, which would naturally be reversed in terms of logic precisely because of this PR, and cannot be easily fixed by simple text replacement.