test-nm-client: fix the remaining counter

test-nm-client: fix the remaining counter

We're expecting four callbacks: a client::devices change,
client::active-connections change, client::activate callback,
and a device::active-connection change.

We only hook the second one in the callback to the first one, and
only if client::active-connections is not set already. If it is
(when running slowly in valgrind), we just decrement the counter.

However, as the counter is one less than it should be, it would
underflow and we wait forever* instead.

For the value of forever=20s, given that's the timeout of the
mockup service.