Present the result, when the script passes unexpectedly

Author: pradyunsg

tests/lib/__init__.py

@@ -666,7 +666,7 @@ def run(
         if expect_error and not allow_error:
             if result.returncode == 0:
                 __tracebackhide__ = True
-                raise AssertionError("Script passed unexpectedly.")
+                raise AssertionError(f"Script passed unexpectedly:\n{result}")
 
         _check_stderr(
             result.stderr,