[gkc]

- What I did
test: fix flaking AtClientTest.test_get_at_keys

- How I did it

• Its own keys: every key the test creates on amateur93 carries a unique per-run token, so overlapping CI runs can't see each other's keys. It will cover a self key, a public key and a key shared with 27barracuda, using only at_python's own API.
• The same three checks as before: a scan for "" returns something; a scan for the token returns exactly the set of names it created; a scan for a token that matches nothing returns nothing.
• Cleanup that always runs: each key's delete is registered as soon as the key is created, so it runs even if an assertion fails or creation stops partway. Each key also gets a short (10-minutes) ttl, so a failed cleanup won't pile up data on amateur93.

- How to verify it
Tests pass