tests: fix test_changing_topic_retention_with_restart

This test was assuming that segment sizes apply exactly,
but they are actually subject to a +/- 5% jitter.  To reliably
remove the expected number of segments, we must set our retention
bytes to 5% less than the amount we really expect to retain.

Fixes redpanda-data#2406

tests/rptest/tests/retention_policy_test.py

@@ -91,10 +91,15 @@ def test_changing_topic_retention_with_restart(self):
         # Wait for controller, alter configs doesn't have a retry loop
         kafka_tools.describe_topic(self.topic)
 
+        # When predicting segment sizes, we must account for segment_size_jitter
+        # which adjusts segment sizes up or down by 5%
+        segment_size_jitter_factor = 1.05
+
         # change retention bytes to preserve 15 segments
         self.client().alter_topic_configs(
             self.topic, {
-                TopicSpec.PROPERTY_RETENTION_BYTES: 15 * segment_size,
+                TopicSpec.PROPERTY_RETENTION_BYTES:
+                int((15 * segment_size) / segment_size_jitter_factor),
             })
         wait_for_segments_removal(redpanda=self.redpanda,
                                   topic=self.topic,