[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>
diff --git a/src/kudu/integration-tests/memory_gc-itest.cc b/src/kudu/integration-tests/memory_gc-itest.cc
index eb9c963..8851884 100644
--- a/src/kudu/integration-tests/memory_gc-itest.cc
+++ b/src/kudu/integration-tests/memory_gc-itest.cc
@@ -92,7 +92,7 @@
     workload.Setup();
     workload.Start();
     ASSERT_EVENTUALLY([&]() {
-      ASSERT_GE(workload.rows_inserted(), 1000000);
+      ASSERT_GE(workload.rows_inserted(), 100000);
     });
     workload.StopAndJoin();
   }