PopulateLogTile error in MySQL storage tests

Here's a snippet from the output of `go test -v ./storage/mysql`:
```
=== RUN TestNodeRoundTrip/store-all-read-all-norevisions
E1028 16:50:59.282101 144538 subtree_cache.go:122] PopulateLogTile(): log repop got: 254 internal nodes, want: 0
=== RUN TestNodeRoundTrip/store-all-read-all-revisions
E1028 16:50:59.299517 144538 subtree_cache.go:122] PopulateLogTile(): log repop got: 254 internal nodes, want: 0
```

By trial and error I've found that these `PopulateLogTile` errors started occurring after [this commit](https://github.com/google/trillian/commit/d7e8fd6655c454c8b37b1f673bf87a443e17fe87). However, that commit's description seems to suggest that it may have only surfaced the issue rather than created it.