Add simple unit tests

[gavv]

Currently we have no unit tests at all.

For some of the components developing a correct test wouldn't be trivial. However we can at least start with adding tests for a few components that can be covered easily:

• MovMax
• MovAvg
• MovDev
• MovGradient
• MovPercentile
• SchmittTrigger

I suggest to implement tests using Google Test library. We'll have to add it to 3rdparty directory as a submodule and add to CMake like we do it here: https://github.com/roc-streaming/rt-tests/blob/master/CMakeLists.txt#L5-L25

We should also add test step to CI.

[saneck2150]

Can i try to do this task?

[gavv]

You're welcome :)

[gavv]

Landed.