rpc_test: Fix test_stream_connection_error with valgrind
rpc_test: Fix test_stream_connection_error with valgrind

There is no synchronization between a connection failing and sink()
producing an error.

In a debug build I normally see an error after 50 iterations, but with
valgrind it is normal to need 650.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200511182655.49101-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar