Set automated tests

Set automated tests using Pytest.

You can use AI to help you write boilerplate code.
But don't forget to test all boundary conditions in the tests.
Also pls double check if the code is working properly or not

The issue is to set up automated tests for a command-line password manager using Pytest. First, examine the existing codebase to understand the modules and functions that need testing, such as password encryption, storage, and CLI commands. Write test files that cover normal operation and edge cases, then run them with Pytest to ensure they pass. The work is done when a test suite is created and all tests pass, verifying the manager's functionality.

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