Add tests for infix_2_postfix() in infix_to_prefix_conversion.py

[hollowcrust]

Describe your change:

Related to #10063.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

We need one pull request that ONLY adds doctests and does not modify the code. That will define the AS-IS behavior. We will merge this one first.

The second will include those doctests modified to the proposed TO-BE code.

[hollowcrust]

Hi I see what you mean now, but the problem is the original code produces outputs with trailing spaces, so the "as-is" doctests cannot pass the current ruff check.

That's why I have to change the code to change the output format with no trailing spaces.

[cclauss]

OK... This is the correct tests for the current code. Please rebase the other pull request AFTER this one is merged.