docs: Mention how to run tests on host with external modules

This works also on Mac OS

Author: jonnor

docs/developing.md

@@ -50,12 +50,27 @@ Install Python packages
 pip install -r requirements.txt
 ```
 
+#### Run tests on PC using external modules build
 
-#### Run tests on PC
+This runs tests by building the modules as external C modules,
+which bakes them into the firmware image/executable.
+
+NOTE: Tested on Linux and Mac OS. Not tested on Windows Subsystem for Linux (WSL).
+
+```
+make check_unix
+```
+
+#### Run tests on PC using dynamic native modules
+
+This runs tests by building as dynamic native modules (.mpy files),
+and the .mpy files are then loaded at runtime.
+
+NOTE: This does not work on Mac OS. Due to https://github.com/micropython/micropython/issues/5500
 
 NOTE: Requires `micropython` program to installed (MicroPython Unix port).
 
-To build and run tests on host
+To build and run tests of dynamic native modules on host
 ```
 make check
 ```