sort the standard library test directory `tests/stdlib`

[saem]

stdlib test category (as in the directory /tests/stdlib) doesn't mirror the structure of the actual code under /lib.

Inconsistencies like these are just painful, the structure being mirrored is poor, but at least then there is only one mistake, the one poor structure. Right now, we've got one poor structure, another questionable structure in the test directory, and a third which is the mapping, or lack thereof, between the two.

Tests are servile, they follow the systems they verify/validate. The current organization has people guessing where could tests be, or where they should go.

The task is simple, arrange tests in the stdlib category in a directory structure that largely mirrors the lib directory. This should be done in series of small PRs.

Once this work is complete, properly organizing the standard library and its tests can take place.