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.
I made a mistake merge. Reverting it in c778945806 undid the state
on master, but now I realize it crippled the git merge mechanism.
As the merge contained a mix of commits from `master..staging-next`
and other commits from `staging-next..staging`, it got the
`staging-next` branch into a state that was difficult to recover.
I reconstructed the "desired" state of staging-next tree by:
- checking out the last commit of the problematic range: 4effe769e2
- `git rebase -i --preserve-merges a8a018ddc0` - dropping the mistaken
merge commit and its revert from that range (while keeping
reapplication from 4effe769e2)
- merging the last unaffected staging-next commit (803ca85c20)
- fortunately no other commits have been pushed to staging-next yet
- applying a diff on staging-next to get it into that state
