test: add unit tests for main and actuation receivers

Exhaustive test cases for both classes, with 100% coverage. Adds `mock` dependency. Tests for actuation receiver do not yet pass.

ref: N25B-168

README.md

@@ -84,7 +84,32 @@ Then resume the steps from above.
 
 ## Usage
 
+On Linux and macOS:
+
 ```shell
-cd src
-python -m robot_interface.main
+PYTHONPATH=src python -m robot_interface.main
+```
+
+On Windows:
+
+```shell
+$env:PYTHONPATH="src"; python -m robot_interface.main
+```
+
+With both, if you want to connect to the actual robot (or simulator), pass the `--qi-url` argument.
+
+
+
+## Testing
+
+To run the unit tests, on Linux and macOS:
+
+```shell
+PYTHONPATH=src python -m unittest discover -s test -p "test_*.py" -v
+```
+
+On Windows:
+
+```shell
+$env:PYTHONPATH="src"; python -m unittest discover -s test -p "test_*.py" -v
 ```