Fix FIFO compaction picker test

Summary:
A FIFO compaction picker test is accidentally testing against an instance of level compaction picker.
Closes https://github.com/facebook/rocksdb/pull/2641

Differential Revision: D5495390

Pulled By: sagar0

fbshipit-source-id: 301962736f629b1c499570fb504cdbe66bacb46f

db/compaction_picker_test.cc

@@ -512,7 +512,7 @@ TEST_F(CompactionPickerTest, NeedsCompactionFIFO) {
         kFileSize, 0, i * 100, i * 100 + 99);
     current_size += kFileSize;
     UpdateVersionStorageInfo();
-    ASSERT_EQ(level_compaction_picker.NeedsCompaction(vstorage_.get()),
+    ASSERT_EQ(fifo_compaction_picker.NeedsCompaction(vstorage_.get()),
               vstorage_->CompactionScore(0) >= 1);
   }
 }