Fix flakiness in column_family_test

Summary:
Fixes #6840824, running "make check" on centos6 hits
a deadlock in column_family_test

Test Plan:
seq 10000 | parallel --gnu --eta 't=/dev/shm/rdb-{}; rm -rf
$t; mkdir $t && export TEST_TMPDIR=$t; ./column_family_test > $t/log-{}'
Made the test deterministic by narrrowing the window for the flush.

Reviewers: igor, meyering

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D38079

db/column_family_test.cc

@@ -724,7 +724,9 @@ TEST_F(ColumnFamilyTest, DifferentWriteBufferSizes) {
   WaitForFlush(1);
   AssertNumberOfImmutableMemtables({0, 0, 0, 1});
   ASSERT_EQ(CountLiveLogFiles(), 5);
-  PutRandomData(3, 90*6, 1000);
+  PutRandomData(3, 240, 1000);
+  WaitForFlush(3);
+  PutRandomData(3, 300, 1000);
   WaitForFlush(3);
   AssertNumberOfImmutableMemtables({0, 0, 0, 0});
   ASSERT_EQ(CountLiveLogFiles(), 12);