test: convert to pytest

Instead of built-in `unittest`, now use `pytest`. Find versions that work, convert tests.

ref: N25B-168

README.md

@@ -105,11 +105,15 @@ With both, if you want to connect to the actual robot (or simulator), pass the `
 To run the unit tests, on Linux and macOS:
 
 ```shell
-PYTHONPATH=src python -m unittest discover -s test -p "test_*.py" -v
+PYTHONPATH=src pytest test/
 ```
 
 On Windows:
 
 ```shell
-$env:PYTHONPATH="src"; python -m unittest discover -s test -p "test_*.py" -v
+$env:PYTHONPATH="src"; pytest test/
 ```
+
+### Coverage
+
+For coverage, add `--cov=robot_interface` as an argument to `pytest`.