Add progress update test to validate advance behaviour

[r-saba]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

The advance branch in the update method of the Progress class was not covered by tests. This PR adds a dedicated test case to validate that advance correctly updates the task's progress.

Prior to this change, modifying the following code did not trigger test failures, meaning potential regressions could go unnoticed:

            if advance is not None:
                task.completed += advance