fix(nimbus): Approve only our own Kinto review in the selenium teardown

[jaredlockhart]

Because

• The selenium teardown called approve() on the kinto_client factory, so it raised AttributeError and was swallowed by the bare except, leaving a pending review that a later test signs as if it were its own.
• approve() checks only the collection status, so any caller signs whatever is pending, and the teardown runs for tests that xdist grouping deliberately keeps off that collection's worker.
• approve() retries for 120s, so activating the teardown unbounded would burn that budget per test whenever no review is pending.

This commit

• Calls the factory so the teardown approves via a KintoClient instance.
• Gives KintoClient a slug and only signs when the pending review changes that slug's record, comparing the workspace and main buckets.
• Threads the test's experiment slug through the kinto_client fixture so every call site is scoped to its own experiment.
• Bounds the teardown to a short explicit retry budget.

Fixes #16978

[jaredlockhart]

Still trying to track down these intermittent Kinto approve CI failures, iterating in this PR with some extra logging.