selftests: mlxsw: sch_red_core: Increase backlog size tolerance

selftests: mlxsw: sch_red_core: Increase backlog size tolerance

Backlog fluctuates on Spectrum-4 much more than on <4. In practice we can
sample queue depth values going from about -12% to about +7% of the
configured RED limit. The test which checks the queue size has a limit of
+-10%, and as a result often fails. We attempted to fix the issue by
busywaiting for several seconds hoping to get within the bounds, but that
still proved to be too noisy (or the wait time would be impractically
long). Unfortunately we have to bump the value tolerance from 10% to 15%,
which in this patch do.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Link: https://patch.msgid.link/f54950df2a8fcba46c3ddc1053376352fa2e592b.1728316370.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>