Tests: Print stack trace if CLI command excepts with run_cli_command

Before, the test would just fail and say that an exception was raised
but it would not display the actual exception, making it difficult to
debug the problem. In the case of a non-zero exit code, the stderr is
printed as well.

tests/conftest.py

@@ -536,8 +536,9 @@ def factory(
             assert result.exception is not None, result.output
             assert result.exit_code != 0, result.exit_code
         else:
-            assert result.exception is None, result.output
-            assert result.exit_code == 0, result.exit_code
+            import traceback
+            assert result.exception is None, ''.join(traceback.format_exception(*result.exc_info))
+            assert result.exit_code == 0, (result.exit_code, result.stderr)
 
         return result