Fix `-f` tests for `visudo`

[pvdrz]

• Fix the flag_file::creates_sudoers_file_with_default_ownership_and_perms_if_it_doesnt_exist test
• Fix the flag_file::flag_has_precedence_over_positional_argument test
• Add API to parse sudoers from a reader directly
• Fix permissions for flag_file::regular_user_can_create_file

Describe the changes done on this pull request This PR changes visudo so it passes all the compliance tests with the exception of the regular_user_cannot_update_a_file_they_dont_own because it deserves a deeper look.

Pull Request Checklist

• [x] I have read and accepted the code of conduct for this project.
• [x] I have tested, formatted and ran clippy over my changes.
• [x] I have commented and documented my changes.
• [x] This pull request will partially fix https://github.com/memorysafety/sudo-rs/issues/657 where a proper discussion about a solution has taken place.

[github-actions[bot]]

Number of dependencies and binary size impact report

Metric	main	PR #681	Delta
Direct dependencies	3	3	-
Total dependencies	4	4	-
Binary size	988.1 KiB	988 KiB	-
Text size	576.5 KiB	576.6 KiB	-

Dependencies diff

```diff └─ sudo-rs [v0.2.0-dev.20230703] ├─ glob [v0.3.1] ├─ libc [v0.2.147] └─ log [v0.4.19] ```