Implement integration test suite for facetec-api-client

[MOZGIII]

As a follow-up after #1, we need a test suite for our facetec-api-client to actually ensure that it works properly with the FaceTec Server SDK.

The way to do it is to use a cloud-hosted FaceTec Test Server. It requires a license key, which we have to keep private, so we can enable it in CI via secrets (i.e. env vars).

The actual test code is intended to be calling the Test Server SDK with a certain test scenario (i.e. a sequence of clean-up, enroll, 3d-enroll, 3d-match).

https://dev.facetec.com/api-postman-test-suite will come in handy.

We have to be careful to not publish any confidential data FaceTec shared with us.

[MOZGIII]

We don't really need to test the API client itself, but rather its integration into our robonode logic.
Integration tests for the robonode logic have already been implemented as part of #39.