Unit tests

[arthurrichards77]

Need full implementation of unit tests for leaf_nodes and idioms.

[arthurrichards77]

Example: in the testing folder next door to drone_trees and examples, have a test_leaf_nodes.py including a test_xxxx function for each leaf node. Each one would use a SITL to verify the leaf node doing what it should.

Slightly harder for flight_idioms.py where the tests are more about logic than function. These could just use dummy leaf nodes. This logic is at the heart of the research project.

[hiradg]

Each leaf node has its corresponding unit test in test_leaf_nodes.py. SITLs for each unit test are presented as a pytest fixture containing start-up and tear-down of the SITL which happens before and after calling the test function. Fixtures are all located inconftest.py and do not need importing.