self_test: to ease debugging, extend error message when tests fail

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

Author: troglobit

self_test/run.py

@@ -67,7 +67,7 @@ def run(self, workers, args=None, expected_return=0, grep=None):
             print(result.stderr, file=sys.stderr)
 
         # Ensure the return code is as expected
-        assert result.returncode == expected_return, f"Failed: Got unexpected retun code {result.returncode}"
+        assert result.returncode == expected_return, f"Failed: Got unexpected return code {result.returncode}. stderr: {result.stderr}"
 
         # If grep is provided, ensure the output contains the specified text
         if grep: