How to add tests for phase 3 entry requirements

[yuri91]

As discussed in https://github.com/WebAssembly/meetings/blob/main/main/2021/CG-06-08.md , it is not obvious how to add tests for the branch hinting proposal.

A suggestion that I think makes sense is to add to the reference interpreter the ability to parse and verify the correctness of the brancHints section, and error out in case of errors (probably only if requested by a command line options).

Other suggestions are welcome.

[yuri91]

This is solved by #20