The test was trying to wait for the replica to start loading the rdb
from the master before it kills the master, but it was actually waiting
for ROLE to be in "sync" mode, which corresponds to REPL_STATE_TRANSFER
that starts before the actual loading starts.
now instead it waits for the loading flag to be set.
Besides, the test was dependent on the previous configuration of the
servers, relying on the fact the replica is configured to persist
(either RDB of AOF), now it is set explicitly.
(cherry picked from commit 37cf8d8f7faedce704d4c18319e6bf72703e1ddc)
