[test] deflake more for memory_gc-itest

In DEBUG slow mode, write workload may not complete before
ASSERT_EVENTUALLY timeout, we have to write less data.

Change-Id: If26004dd87c5dee68c9e1f31f0dd3caccc938ecf
Reviewed-on: http://gerrit.cloudera.org:8080/14480
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <adar@cloudera.com>
1 file changed