Fix/es6 tests - Refactor(tests): convert Department and Instructor test files to ES6 modules

[agaesh]

Title: refactor(tests): convert Department and Instructor test files to ES6 modules

Description:
This pull request refactors the DepartmentService.test.js and InstructorService.test.js files to use pure ES6 module syntax.

Changes:

• Replaced all require statements with import.
• Updated service and model imports to ES6 syntax.
• Ensured all mocks and test logic remain unchanged.
• Made paths and module references ES6 compatible.

Notes:

• This change is part of the ongoing ES6 migration effort.
• Integration testing will be handled later in the es6-migration branch.

Branch Name:
fix/es6-test-files

[agaesh]

This pull request closes issue #75 as the DepartmentService.test.js and InstructorService.test.js files have been successfully converted to ES6 modules.