Add EthernetTestInterface to list of created interfaces

This is a bug that was missed in my last CL. Newly created
EthernetTestIntefaces need to be added to the createdIfaces list, so
they are properly destroyed on tearDown.

Test: atest CtsNetTestCases:EthernetManagerTest
Change-Id: I53bd728a7ecb5a37ae89079ade0a3ebdae304a1a
1 file changed