tests: wait for acks in upgrade test

Wait for at least some of the messages to be successfully written to
topic before starting consumers to prevent them trying to read empty
partition.

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit 6823527)

tests/rptest/tests/upgrade_test.py

@@ -226,6 +226,9 @@ def test_upgrade_with_all_workloads(self, single_upgrade):
                 # First version, start up the workload
                 self._producer.start(clean=False)
                 self._producer.wait_for_offset_map()
+                self._producer.wait_for_acks(100,
+                                             timeout_sec=10,
+                                             backoff_sec=2)
                 wrote_at_least = self._producer.produce_status.acked
                 for consumer in self._consumers:
                     consumer.start(clean=False)