Address the /code-review findings on the S2 test-net:
- as_user no longer sets a global `lambda: user` override that two users in one
test would collide on; the shared current_user override now resolves the caller
from an x-test-user header, and each returned client tags its own requests, so
as_user(a)/as_user(b) are independent (#1).
- override_dep records the prior value per dependency key and restores it on
teardown instead of a blanket dependency_overrides.clear(), so fixtures compose
without clobbering each other (#5).
- a _configured fixture bakes the post-setup state into client/as_user, and the
per-test inline monkeypatch / autouse duplicates are removed (#4).
- the feed keyset walk asserts all seeded rows surfaced before comparing, so a
seeding regression can't make the sorts pass vacuously (#3).
peter
e764ca4568