Refactor tests for truth maintenance into new test_truth_maintenance.cljc file

[WilliamParker]

As the name suggests, I'm envisioning this file containing tests that validate that truth maintenance works. In terms of areas of code tested, while the tests would be tests of overall correctness (as is our current pattern) areas particularly covered by this test file would include but not be limited to ProductionNode's left-retract, add-insertions!, and remove-insertions!.

[WilliamParker]

I merged #319 which addresses this. Closing.