Don't hard code the PK of objects in add-tests

The ID of newly created objects doesn't need to be hard coded, it can be
retrieved from the database. This simplifies a bit the testcases for the
'add' tests, as the pk-attribute is now obsolete.
Simplify loop by using list comprehensions
Move common code to helper function
Dynamically add test methods for the delete tests

This makes sure that there is an individual test function for eath test user
and guarantees a re-setted database each time. For this to work, it is
necessary to call a helper function and pass it the class.
First implementation of documents for gym members