Adding unit tests

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The project size is increasing. The unit tests should be added for the following parts, with the following priority:

• Display driver and SPI bus driver

• Date time service interaction and synchronizing via BLE

• Event dispatcher

• Clock Page Handler

• Buttons driver

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Also to

• Main window

• Controllers classes

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https://github.com/google/googletest/blob/master/googletest/docs/primer.md

[ValentiWorkLearning]

https://github.com/google/googletest/blob/b8e2562086ef326a4a88089f85b29ce7b6f9464b/googlemock/docs/ForDummies.md

[ValentiWorkLearning]

https://github.com/google/googletest/blob/master/googlemock/docs/cheat_sheet.md

[ValentiWorkLearning]

https://github.com/google/googletest/blob/master/googletest/docs/advanced.md