selftests: drv-net: pp_alloc_fail: lower traffic expectations

selftests: drv-net: pp_alloc_fail: lower traffic expectations

Lower the expected level of traffic in the pp_alloc_fail test
and calculate failure counter thresholds based on the traffic
rather than using a fixed constant.

We only have "QEMU HW" in NIPA right now, and the test (due to
debug dependencies) only works on debug kernels in the first place.
We need some place for it to pass otherwise it seems to be bit
rotting. So lower the traffic threshold so that it passes on QEMU
and with a debug kernel...

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251007232653.2099376-9-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>