Update chain synchronizer tests in the node to cover the trie leaf chunking scenario

The chunking mechanism is only tested indirectly via `casper-test`.

We should extend the tests in node ([`test_chain.rs`](http://test_chain.rs/)) to cover trie chunking, so it is tested on CI for each PR.

The test should focus on the `fetch_trie_retry_forever` and exercise the code path associated with the `TrieOrChunk::ChunkWithProof` variant being retrieved, however, there might be more tests needed for the `chain_synchronizer` module.