Update tdd section

    Update TDD section in 05-defensive programming
    
    The reason for this change is to encourage users not to write a test function
    with many assertions, but rather a separate test function for each assertion.
    
    This makes it easy to introduce testing frameworks and is here illustrated by
    calling nosetests when the test functions have been saved to a file.

    I have edited the nosetests output and removed explicit references to my local home directory