Stabilize DBTest.ApproximateSizesMemTable (#7064)

Summary:
Random memtable layouts could cause random failure,
reproducible with command below running for a while. Test now using
deterministic behavior.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7064

Test Plan: while ./db_test --gtest_filter=*SizesMemTable*; do true; done

Reviewed By: siying

Differential Revision: D22339442

Pulled By: pdillinger

fbshipit-source-id: 8e74e5a9b5e88f7030854045a22c12cf561d5de6

db/db_test.cc

@@ -1421,7 +1421,9 @@ TEST_F(DBTest, ApproximateSizesMemTable) {
     keys[i * 3 + 1] = i * 5 + 1;
     keys[i * 3 + 2] = i * 5 + 2;
   }
-  RandomShuffle(std::begin(keys), std::end(keys));
+  // MemTable entry counting is estimated and can vary greatly depending on
+  // layout. Thus, using deterministic seed for test stability.
+  RandomShuffle(std::begin(keys), std::end(keys), rnd.Next());
 
   for (int i = 0; i < N * 3; i++) {
     ASSERT_OK(Put(Key(keys[i] + 1000), RandomString(&rnd, 1024)));