Consider removing the byte level unit tests for pkgs/shares

[rootulp]

Context

pkg/shares currently contains many byte level unit tests for share encoding.

Problem

The unit tests are brittle and require manual updating every time we change share encoding.

Proposal

Consider isolating the byte level unit tests to the share builder. If we unit test the share builder exhaustively, then higher level abstractions (i.e. other files in pkg/shares) can test via the exposed methods on a Share rather than the raw bytes.