While looking at tests/gnrc_ipv6_ext_frag again while writing
RIOT-OS/Release-Specs#137, I noticed that several of tests that I
definitely wrote myself from scretch are attributed wrong (and
sometimes even documented wrong). I guess this was caused by just
copy-pasting the files...
Use a minimalistic dummy instead of the default interface for testing
the `gnrc_ipv6_ext` module.
Currently the default interface is used which leads to problems with
this test, since random traffic on the medium or a missing default
interface might lead to failed results.
Since the `tap` dependency is removed for `native`, I add this test for
testing on CI.
