tstime/rate: deflake TestLongRunningQPS even more

Previous de-flakings:
* 8cf1af8a07 for #3733
* 30458c71c8 for #2727

Fixes #4044

Change-Id: I506cf1ff37bb224f5a9929f1998901e60b24535d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f