Add unit tests for C library

[gavv]

After implementing #33, we'll also need simple unit tests for the new C API.

The test can do the following:

• create estimator
• get playback (output) samples from it
• put the same samples as captured (input) samples to it
• get and check latency reports

The test can push input samples with different shifts and check that reported latency is changed accordingly.

[saneck2150]

Hello, I would like to work with this task. This will be my first time working on your project, please advise where I should start

[gavv]

@saneck2150 Hi! As mentioned in description, this one can be done only after completing #33, which is not implemented so far.