This was already too small in 6c2510dc so it should have been properly
tested. Now that we are testing it on CI with binaries compiled by LLVM
this leads to crashes with this "new" platform.
Use a minimalistic dummy instead of the default interface for testing
the `gnrc_sixlowpan` module
Currently the default interface is used which leads to problems with
this test, since random traffic on the medium might lead to failed
results.
