Add unit tests for A* path planner (PythonRobotics #123)

[rsleiti]

What does this implement/fix?

This pull request adds two unittest test cases for the A* path planner in PathPlanning/AStar/a_star.py. The tests check basic path generation and the edge case where the start equals the goal.

Why is this useful?

This improves testing coverage for a core algorithm in the PythonRobotics repository. It helps ensure the planner behaves correctly after future changes.

Checklist:

• [x] Added meaningful unittests
• [x] Tests pass with python3 -m PathPlanning.AStar.test_astar_unit