opam list bench & reftest

[rjbou]

Current list test (list.test and in the future list-xxx.test see #5375) was written before bench CI with the purpose of benching performance improvements (cf. #5375 (comment)).
We should add instead add bench test for depends-on & required-by performance test, and write a small ones for functional depends-on and required-deps options (and he other tested).