Update rtsock_l3 test after 2fe5a79425c79f7b828acd91da66d97230925fc8

[arichardson]

Summary

It appears that we now get a different errno. Looking at Jenkins to find
the first commit where this test starting failing indicates that
2fe5a79425c79f7b828acd91da66d97230925fc8 is the most likely cause.

Test Plan

test now passes

[melifaro]

tests/sys/net/routing/test_rtsock_l3.c

399–400

In reality, EEXIST is a previous value from some of the system calls, and the real errno result is 0.
This test should be renamed to something like rtm_get_v4_hostbits_success - didn't have cycles to do so yet.

[arichardson]

Update to make them success tests