add unit tests

[markkaylor]

What does it do

Adds unit tests for #23

How to test it

Confirm the unit tests provide adequate coverage for the PR mentioned above.

That PR was tested as follows:

• Clone this v3 repo that has all the relations for a content-type https://github.com/markkaylor/strapi-v3-test-migration
• Clone this v4 repo https://github.com/markkaylor/strapi-v4-test-migration
• On the v3 app run the codemods scripts ../path/to/codemods/bin/cli.js migrate:application <v3-application-path>
• Compare the relations to the v4 repo's relations
• Confirm all files and folders were moved correctly based on https://docs.strapi.io/developer-docs/latest/setup-deployment-guides/file-structure.html

[markkaylor]

I also have a question about the way getRelationObject was implemented in the previous PR. What happens if I have a relation pointing to a plugin's content type

Good question and good catch! I definitely overlooked that and it doesn't look like bi-directional relations are handled correctly for plugins. I will try to fix that this afternoon or tomorrow.