crc16: add tests

[leighmcculloch]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Add tests to the crc16 package.

Why

We don't have any. I was digging around in the code for some thing else and wanted to make a change to the crc logic and make sure I don't break it.

Known limitations

N/A

[leighmcculloch]

Dependencies upgraded:

• 0 github.com/stretchr/objx v0.1.1 > v0.3.0 stretchr/objx@v0.1.1...v0.3.0

• 1 github.com/stretchr/testify v1.6.1 > v1.7.0 stretchr/testify@v1.6.1...v1.7.0

[acharb]

Lgtm!