tests: net: ethernet_mgmt: move to new ztest API

Move net/ethernet_mgmt tests to use new ztest API
1. Combine the tests of the same kind
2. To make the test be able to run multiple times
   set back "auto neg" to original state(true) when test is done.
   set back linkspeed to original 10Mbps when the test is done.
   set back duplex to original state(true) when test is done.

Signed-off-by: Hu Zhenyu <zhenyu.hu@intel.com>