Unit tests for TaskList

[Merlissa09]

As a developer, I want automated tests for TaskList so that I can confidently make changes without breaking core functionality.

• Tests cover addTask(), removeTask(), getPendingTasks(), and getCompletedTasks()
• At least one test verifies what happens when trying to add a task that doesn't exist
• At least one test verifies what happens when trying to delete a task that doesn't exist
• Tests should be written to verify invalid input
• At least one task should verify the behavior of getPendingTasks when there are no pending tasks.
• At least one test verifies that marking a task complete moves it from pending to completed
• All tests pass with no manual console inspection require