tests: net: ethernet_mgmt: Fix tests when running in real hw
tests: net: ethernet_mgmt: Fix tests when running in real hw

If the test is run in a board that has a network interface
enabled, then the tests might select and use wrong network interface
that is needed for the test. So this commit makes sure that
we always use the simulated network interface for the
Ethernet management tests.

Fixes #36419

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr