Add -k for testing a specific example in the test readme

Author: M65649

test/readme.md

@@ -9,5 +9,5 @@ Install the python packages needed: `pip install -r requirements.txt`
 ## Running
 
 * Run all tests: `pytest . --port <device_port> -s`
-* Run a test for a specific example sketch: `pytest . --port <device_port> -s test_<sketch_name_without_dot_ino>`. For example for `debug_modem`: `pytest . --port COM7 -s test_debug_modem`
+* Run a test for a specific example sketch: `pytest . --port <device_port> -s -k test_<sketch_name_without_dot_ino>`. For example for `debug_modem`: `pytest . --port COM7 -s -k test_debug_modem`