Fix failure in rocksdb unit test CompressedCache

Summary:
The problem was that there was only a single key-value in a block
and its compressibility was less than 88%. Rocksdb refuses to
compress a block unless its compresses to lesser than 88% of its
original size. If a block is not compressed, it does nto get inserted
into the compressed block cache.

Create the test data so that multiple records fit into the same
data block. This increases the compressibility of these data block.

Test Plan: ./db_test

Reviewers: kailiu, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13905

db/db_test.cc

@@ -1800,7 +1800,7 @@ TEST(DBTest, CompressedCache) {
     std::string str;
     for (int i = 0; i < num_iter; i++) {
       if (i % 4 == 0) {        // high compression ratio
-        str = RandomString(&rnd, 100000);
+        str = RandomString(&rnd, 1000);
       }
       values.push_back(str);
       ASSERT_OK(Put(Key(i), values[i]));