pytests/test_modem.py: fix aplay -l output capturing

I get the following error on the L5:

TypeError: argument of type 'CompletedProcess' is not iterable

and using out.stdout solves the problem.
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>

pytests/test_modem.py

@@ -25,7 +25,7 @@ def test_modem_alsa():
     out = subprocess.run(["aplay", "-l"], capture_output=True)
     assert(out.returncode == 0)
     if boardtype.is_librem5():
-        assert(b"gtm601" in out)
+        assert(b"gtm601" in out.stdout)
     else:
         assert(b": SIM7100 [SIMCom SIM7100]" in out.stdout)