Change delete function signature to match new.

The wp_test_suite_delete and wp_test_suite_test_delete function
signatures were changed to match the signatures of their *_new
counterparts. Deallocating a pointer from either function now requires a
reference, i.e. wp_test_suite_delete(&suite). Function calls were
updated across the source file to account for the change.

Additionally:

 - Further improved documentation of some of the suite allocator and
   deallocator functions.

 - Broke some larger functions into documented named-sections of more
   digestable pieces.

 - Improved linked-list code.

 - Improved error handling of failed malloc calls. Functions may now
   return NULL as a valid value.