serialize netconf tests

Netconf has both unit tests and shell tests which interfere badly when run in parallel.

The obvious round about is to not call make check with a -j parameter including make distcheck.

IMHO the problem belongs in Sysrepo which was not designed to support parallel activities on same objects.

If nobody (Stephen?) proposes something simpler I'll put the shell script into a separated directory.

Note I leave the classification of this to QA, i.e. is the constraint to not run make distcheck in parallel is a problem.