Rewrite scripts/test.sh into actual Rust tests

[radu-matei]

One of the goals of this implementation is to be fully compatible with the Go implementation of a canonical JSON serializer used today in Docker and TUF/Notary, and scripts/test.sh is an early attempt at making sure the SHA256 sum of files generated by this implementation matches the one of files generated by the Go implementation.

However, the current Bash implementation is not ideal, and should be reworked into Rust tests - in order to achieve this:

• add the canonical JSON representation of the files in testdata into testdata/canonical
• the Rust tests would read the initial JSON, serialize in in canonical form, and calculate the SHA 256 sum of the output, then compare it with the Go implementation.