U-Boot crashes when doing a 'ping' with the following test scenario:

  - All ethernet devices are not probed
  - "ethaddr" for all ethernet devices are not set
  - "ethact" is set to a valid ethernet device name

Add a new test case 'dm_test_eth_act' to hit such scenario.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>