REGTESTS: sample_fetches: pin one thread in the connection address test

REGTESTS: sample_fetches: pin one thread in the connection address test

The test asserted that dst_conn reports exactly one connection, which
holds when it runs alone but not when the reg-tests are run in parallel
on a loaded machine: it was reported failing with a value of 8.

The extra connections are not real. With several threads accepting in
parallel, the process ends up reporting current connections which were
never established: after the only client has left, "show info" reports
a non-zero CurrConns while CumConns has only counted the connections
which really happened, and "show stat" shows the same discrepancy
between scur and stot for the frontend. Forcing nbthread to 1 makes the
counters exact again, which is what this test needs since it checks the
address fetches, not the connection accounting.