Test History Tree serialization

[conradoplg]

Motivation

#2553 adds History Tree serialization in the state using bincode and serde (using a helper HistoryTreeParts struct). However, there is no particular tests for this functionality.

Add tests for it.

See https://github.com/ZcashFoundation/zebra/pull/2553/files#r680669776 for context

Specifications

N/A

Designs

N/A

Related Work

Follow up to #2553

Note that in #2331 we want to remove serde and change the serialization format, which will change the tests.

[teor2345]

This seems ok for now.