Standardize unit tests

[dgmiller]

The format is as follows:

# import statements
import ...

def test_feature_or_function_name():
    """Docstring describing what is being tested."""
    
    # Case 1: The type of test (for example, add two monomials with positive coeffs)
    ...test cases here...
    assert ...
    assert ...
    assert ....

    # Case 2: Another type of test (for example, add two monomials with negative coeffs)
    ...test cases here...
    assert ...
    assert ...
    assert ...

    .
    .
    .