test_run:wait_upstream() and test_run:wait_downstream() now wait until
an upstream / a downstream appears. This prevents an attempt to index a
nil value when one of those functions are called before a record about a
peer appears in box.info.replication. It was observed on
replication/show_error_on_disconnect test after commit
c6bea65f ('replication: recfg with 0
quorum returns immediately').

Fixes #4563.