selftests: forwarding: Mark performance-sensitive tests

When run on a slow machine, the scheduler traffic tests can be expected to
fail, and should be reported as XFAIL in that case. Therefore run these
tests through the perf_sensitive wrapper.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/9a357f8cf34f5ececac08d43a3eb023008996035.1711464583.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>