fix: created mock to override file open

[ian-bowden-slalom]

This PR mocks the builtins.open for the test_that_update_with_new_pattern_succeeds test so that the test doesn't open and modify the actual secureli.yaml file in the repository

Tested that all tests still pass and that the secureli file is no longer modified after running tests

Clean Code Checklist

• [ ] Meets acceptance criteria for issue
• [ ] New logic is covered with automated tests
• [ ] Appropriate exception handling added
• [ ] Thoughtful logging included
• [ ] Documentation is updated
• [ ] Follow-up work is documented in TODOs
• [ ] TODOs have a ticket associated with them
• [ ] No commented-out code included