This fixes a regression introduced in 8b9cad01e9619f53dc5a65892ca6a09ca5de3447
where ovs-vsctl would no longer be available in PATH.
* gnu/tests/networking.scm (run-openvswitch-test): Qualify "ovs-vsctl"
invocation by ungexping the OPENVSWITCH variable instead of assuming it's
available on PATH.