lib_base/test: migrate qcheck alcotest tests to qcheck tezt for skip list

What

In the effort to migrate alcotest test to tezt, this MR migrates Skip_list tests. A tezt tag is added to regroup all lib_base tests.

Why

How

Manually testing the MR

dune exec src/lib_base/test/main.exe -- --file test_skip_list.ml

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR