Add unit tests for TransactionsManager using LocalPersistenceServiceMock

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Description

This PR adds unit tests for the TransactionsManager class in the CashBud project. The unit tests use a mock implementation of the LocalPersistenceServiceProtocol called LocalPersistenceServiceMock. The purpose of these tests is to ensure the correct functionality of the TransactionsManager.

Summary of Changes

Created a new file called LocalPersistenceServiceMock.swift in the CashBudTests/Mocks directory. This file contains a mock implementation of the LocalPersistenceServiceProtocol.
Modified the TransactionsManagerTest.swift file in the CashBudTests/Tests/Managers directory to import the LocalPersistenceServiceMock file.
Added unit tests to the TransactionsManagerTest.swift file to test the functionality of the TransactionsManager using the LocalPersistenceServiceMock.
Set all necessary variables in the LocalPersistenceServiceMock for the unit tests.
Tested the functionality of the TransactionsManager using the LocalPersistenceServiceMock.

Please review and merge this PR to ensure proper testing of the TransactionsManager.

Fixes #65.

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