Instrument tests to write assertions over screen data

**Feature description**

Assertions for emulator screen data

**Is your feature request related to a problem? Please describe.**
During a test run it's not possible to assert that screen is rendering certain information at a given moment in time.

**Describe the solution you'd like**
Instrument tests to write assertions over screen data

**Describe alternatives you've considered**
This feature can be implemented by rendering emulator window in a bitmap and extracting the bits using image processing techniques . This might be more accurate , but is a bit more (and unnecessarily) complicated approach .

**Additional context**
Noticed while trying to write tests for #169 in #172 .

**Possible implementation**
A preliminary roasmap to get this done is as follows

- [ ] Keep internal testing context to track screen draw events including, at least , bitmaps and text.
- [ ] Make this context available to `test_fsm.c` unit test cases
- [ ] Implement debug messages for querying screen state without interfering with statndard firmware message processing, i.e. via `usb_debug`
- [ ] The former implies defining a separate `debug.proto` file in hw-protob (`TODO:` new issue)