selftests/bpf: check for spurious extacks from the driver

Drivers should not report errors when offload is not forced.
Check stdout and stderr for familiar messages when with no
skip flags and with skip_hw.  Check for add, replace, and
destroy.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>