selftests: RED: Use defer for test cleanup

Instead of having a suite of dedicated cleanup functions, use the defer
framework to schedule cleanups right as their setup functions are run.

The sleep after stop_traffic() in mlxsw selftests is necessary, but
scheduling it as "defer sleep; defer stop_traffic" is silly. Instead, add a
local helper to stop traffic and sleep afterwards.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>