rptest: use lower local disk retention for cloud_storage_timing_stres…

…s_test

The test allowed for 10 segments worth of local disk, and would assert
that some data would be read from cloud. With sliding window compaction
on by default, Redpanda compacts more aggressively, which doesn't always
leave room for 10 segments to build up, resulting in no cloud segments
being read.

This updates the test to use a lower local disk retention to encourage
reads from cloud.

tests/rptest/tests/cloud_storage_timing_stress_test.py

@@ -260,7 +260,7 @@ def __init__(self, test_context):
             cloud_storage_idle_timeout_ms=100,
             cloud_storage_segment_size_target=4 * self.log_segment_size,
             cloud_storage_segment_size_min=2 * self.log_segment_size,
-            retention_local_target_bytes_default=10 * self.log_segment_size,
+            retention_local_target_bytes_default=2 * self.log_segment_size,
             cloud_storage_enable_segment_merging=True,
             cloud_storage_cache_chunk_size=self.chunk_size)