Unit tests for traction_innkeeper plugin

[esune]

Most Traction plugins have been moved to https://github.com/hyperledger/aries-acapy-plugins, however the traction_innkeeper plugin will remain in the repository as it is specific to the use-case/application.

The plugin needs unit tests: follow the pattern established in the shared plugin repo and implement tests that can be executed automatically.

[github-actions]

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