More dscheck tests with dscheck.0.3.0

[lyrm]

dscheck.0.3.0 being way faster than the previous version, we now can have more complex dscheck tests, and in a general way, should be less hesitant to add more tests if needed. This is particularly true for data structures with some randomness in it (like skip lists) where we should make sure to have a chance to encounter all cases.

In a general way, for a data structure, we should have :

• testing between all pairs of functions where at least one function to a atomic write operation. For example, for a queue with push, pop, is_empty, we should have at least 5 tests (push-push, pop-pop, push-is_empty, push-pop, pop-is_empty).
• testing all edge cases like on empty or full data-structures
• testing with more than 2 domains, if it may cause situations non reproducible otherwise.

There are probably tests missing for most current data structures.