fakeclient: retry GenerateName on AlreadyExists collisions (match K8s 1.32 behavior)

The real API server retries GenerateName collisions up to 7 times
(RetryGenerateName feature gate, GA in K8s 1.32). The fake client now
does the same, preventing flaky tests caused by birthday-paradox
collisions when many objects share the same GenerateName prefix.

Signed-off-by: Victor Vieux <vieux@repl.it>