test: Lookup for struct net_if interface defined in arp test

The struct net_if object placement in linker 'net_if_area' section
now depends on the order of liked libraries (like
app/libapp.a(main.c.obj). As a result interfaces defined in testing
applications are placed first in the 'net_if_area' section.

Problem arises when board has other interfaces, which are placed on the
first (i.e. default) position. In this case the test just fails as wrong
network interface is used.

Signed-off-by: Lukasz Majewski <lukma@denx.de>