server: reduce flakiness of TestDistributedSweeping.
server: reduce flakiness of TestDistributedSweeping.

Bump 20% the # of sweeps to cleanup all outstanding reminders.

I ran it 1600 times and didn't hit a flake, but it's randomish
by design so we may need to bump it more in the future.

R=tikuta, vadimsh

Fixed: 1142803
Change-Id: Idbbdee3bb1680b1a4ee2c71ef21a6bfca1a96115
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2510289
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go