Fix LITE test build broken by recent commit (#6295)

Summary:
A recent commit adds a unit test that uses a function not available in LITE build. Fix it by avoiding the call
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6295

Test Plan: Run the test in LITE build and see it passes.

Differential Revision: D19395678

fbshipit-source-id: 37b42835bae02511630d80f7cafb1179401bc033

db/db_test2.cc

@@ -4294,7 +4294,9 @@ TEST_F(DBTest2, SameSmallestInSameLevel) {
   ASSERT_OK(db_->Merge(WriteOptions(), "key", "8"));
   Flush();
   dbfull()->TEST_WaitForCompact(true);
+#ifndef ROCKSDB_LITE
   ASSERT_EQ("0,4,1", FilesPerLevel());
+#endif  // ROCKSDB_LITE
 
   ASSERT_EQ("2,3,4,5,6,7,8", Get("key"));
 }