Unit-testing: Consider using mocked responses when using external APIs

[gudmdharalds]

Currently, the unit-tests use external APIs directly during testing. This has a number of downsides:

• External APIs have to be 100% reliant while running unit-tests.
• External data can go inconsistent, leading to unit-test failing.
• The approach costs more resources and takes longer time.
• API calls are sometimes rate-limited, which we have sometimes had problems with.

Originally, unit-tests were implemented this way due to simplicity of this approach. However, it has become less sustainable as the complexity of the project increases and unit-tests get more important.

We should consider implementing mocked API responses in the unit-test suite and move into this direction over time. There should be an option to skip the mocked tests intended for manual running of full test suite.