Tests for v0.1 functionality

[s-kostyuk]

Add missing unit tests for different classes that was present in v0.1 of this application

• [X] Tests for utils
• [X] Tests for auth
• [x] Tests for ApiGateway
• [x] Tests for RestApi
• [x] Tests for integrations infrastructure (Registry)
• [x] Tests for PlacementBuilder
• [x] Integrations: Thing test suite

[s-kostyuk]

• ApiGateway: all methods covered except send_command
• RestApi: all endpoints covered except /messages/

[s-kostyuk]

• Things: In progress
• DummySwitch: Tests done
• TODO: Base TestCases for Actuator and its subtypes

[s-kostyuk]

Implemented two variants of Thing-related unit tests.

• The first one is just a usual set of TestCases, one per each class: https://git.io/vdZvV. Which leads to a huge code duplication.

• The second one contains a set of generic tests of interface implementations. And implementations-specific TestCases that use the generic one. https://git.io/vdZvK

The question is: it is a good practice to use inheritance for TestCases? And is there another way to reduce code duplication for different implementations of the same interface?

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Some food for thoughts:

• The best related SO question: Unit Testing Interfaces in Python
• Why inheritance in unit tests is bad: abstract test case using python unittest
• SO question three: How to write generic unittests for testing plugin classes in python
• How to hide TestCase from discovery: link
• An article with .NET-related solution: Unit Testing Interfaces in .NET

[s-kostyuk]

Implemented in 19562c6. Will be reviewed in the next milestone