Changes

tests: adjust to osmotestconfig no copytree change

openbsc/doc/examples/osmo-bsc_nat/osmo-bsc-nat.cfg references a
bscs.config in the same directory. With a recent change in
osmotestconfig.py, the file does not get copied anymore to the test
directory. Copying additional files around caused problems when running
osmotestconfig.py during 'make distcheck'. (openbsc didn't run into
these issues because we don't run the vty-tests in 'make distcheck'.)

Make the test pass by running it on a copy of the config without:

  nat
   bscs-config-file bscs.config

Related: osmo-python-tests Ic312d546da1c21f68a80b6a188616ef9bc84f4c6
Change-Id: Ice759b2205308ffb827293256eb1c0de9ac67edd