Bugfix aa-decode test: Get stderr correctly, don't ignore stderr contents

Defining 'stderr = subprocess.STDOUT' as a default value for function
did not work and the 'stderr' was always empty, thus also 'outerr' was
always empty and not standard error contents was ever considered in any
way.

Best in fact was to remove excess function arguments as they were not even
used and replace it with a simpler and less error prone structure.

Even after reading 'stderr' correctly it did not help much as all tests
used 'assertIn' which ignored excess output. Better replace the normal
output with the error output if there ever was something, since stderr
is most likely a serious thing and tests should stop on it.

utils/test/test-aa-decode.py

@@ -28,23 +28,33 @@ def subprocess_setup():
     # non-Python subprocesses expect.
     signal.signal(signal.SIGPIPE, signal.SIG_DFL)
 
-def cmd(command, input = None, stderr = subprocess.STDOUT, stdout = subprocess.PIPE, stdin = None, timeout = None):
+# Define only arguments that are actually ever used: command and stdin
+def cmd(command, stdin=None):
     '''Try to execute given command (array) and return its stdout, or return
     a textual error if it failed.'''
 
     try:
-        sp = subprocess.Popen(command, stdin=stdin, stdout=stdout, stderr=stderr, close_fds=True, preexec_fn=subprocess_setup)
+        sp = subprocess.Popen(
+            command,
+            stdin=stdin,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            close_fds=True,
+            preexec_fn=subprocess_setup
+        )
     except OSError as e:
         return [127, str(e)]
 
-    out, outerr = sp.communicate(input)
-    # Handle redirection of stdout
-    if out == None:
-        out = b''
-    # Handle redirection of stderr
-    if outerr == None:
-        outerr = b''
-    return [sp.returncode, out.decode('utf-8') + outerr.decode('utf-8')]
+    stdout, stderr = sp.communicate(input)
+
+    # If there was some error output, show that instead of stdout to ensure
+    # test fails and does not mask potentially major warnings and errors.
+    if stderr:
+        out = stderr
+    else:
+        out = stdout
+
+    return [sp.returncode, out.decode('utf-8')]
 
 
 def mkstemp_fill(contents, suffix='', prefix='tst-aadecode-', dir=None):