selftests: net: ignore timing errors in txtimestamp if KSFT_MACHINE_SLOW

This test is time sensitive. It may fail on virtual machines and for
debug builds.

Similar to commit c41dfb0d ("selftests/net: ignore timing errors in
so_txtime if KSFT_MACHINE_SLOW"), optionally suppress failure for timing
errors (only).

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>