test-util-sockets: Synchronize properly, don't sleep(1)

test-util-sockets: Synchronize properly, don't sleep(1)

The abstract sockets test spawns a thread to listen and accept, and a
second one to connect, with a sleep(1) in between to "ensure" the
former is listening when the latter tries to connect.  Review fail.
Risks spurious test failure, say when a heavily loaded machine doesn't
schedule the first thread quickly enough.  It's also slow.

Listen and accept in the main thread, and start the connect thread in
between.  Look ma, no sleep!  Run time drops from 2s wall clock to a
few milliseconds.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>