test_libnm_linking() executes ./test-libnm-linking which
is supposed to crash. When the user set `ulimit -c unlimited`
before, this will leave a left-over core file.

In case of `make distcheck`, this is quite bad because it lets
the make target fail:

    ERROR: files left in build directory after distclean:
    ./libnm-util/tests/core.31481

Fix this by setting the (soft) rlimit for writing core files
to 0 before executing the test binary.

Signed-off-by: Thomas Haller <thaller@redhat.com>