Rationale: Since RFCs 140 and 146, the old category-based hierarchy is
deprecated and obsolete, and a new approach took place: packages should be as
self-contained as possible.
This paradigm is reflected in many new strict checks that prohibit a package to
refer to files outside its directory tree.
Following this spirit, this commit essentially moves nixpkgs
pkgs/test/default.nix to ./tests/default.nix.
Further, to keep the top-level `tests` attribute, a green alias is kept in the
place of older file.
f72e74d7ae