selftests: net: add more info to error in bpf_offload

selftests: net: add more info to error in bpf_offload

bpf_offload caught a spurious warning in TC recently, but the error
message did not provide enough information to know what the problem
is:

  FAIL: Found 'netdevsim' in command output, leaky extack?

Add the extack to the output:

  FAIL: Unexpected command output, leaky extack? ('netdevsim', 'Warning: Filter with specified priority/protocol not found.')

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>