Add module `test_models`.

The responsibility of this new module is to test all virtual model attributes.
As is, the new module only tests a single method on a single class. However,
this small amount of code provides a template for writing many future tests.
Test method models.Character.__str__

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.fatigue

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.hitpoints

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.alertness

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.will

Add a test for the aforementioned method to module `test_models`. In the process
of testing this method, fix an error in class `Character`.
Test method models.Character.fright

Add a test in module `test_models`.
Test method models.Character.initiative

Add a test in module `test_models`.
Test method models.Character.no_encumbrance

Add a test in module `test_models`. Fix a typo in the method name.
Test encumbrance-related model.Character methods.

Add tests for the following methods:
* light_encumbrance
* medium_encumbrance
* heavy_encumbrance
* extra_heavy_encumbrance

Also fix several spelling errors. Change the word "encumberance" to
"encumbrance".