Try it with explicit returncode

tests/test_webservice.py

@@ -193,7 +193,8 @@ async def test_get_slurm_nice(fake_process, cycle, num_jobs, expected_result):
         ['squeue', '-h', '-o', '%.20j', '-p', 'exfel', '--me'],
         stdout=b'\n'.join(
             [f'correct_SPB_{i}'.encode('ascii') for i in range(num_jobs)] +
-            [f'correct_FXE_{i}'.encode('ascii') for i in range(num_jobs*2)]))
+            [f'correct_FXE_{i}'.encode('ascii') for i in range(num_jobs*2)]),
+        returncode=0)
 
     ret = await get_slurm_nice(
         'exfel', 'SPB', cycle, job_penalty=3, commissioning_penalty=5)