Add unit tests for `TaskAction` and `TaskPlan` objects

[Sebastian Castro]

The TaskAction and TaskPlan objects, which are used to represent actions (or sequences thereof) for task planning and execution, need unit tests.

These can be found in pyrobosim/pyrobosim/planning/actions.py.

As these are relatively simple objects with not much state, the tests should focus mostly on constructing objects and checking that the contents of the objects have the expected values based on the input arguments.