util/helper: fix appending \n to unterminated final streamed chunks in ProcessWrapper

Addressing bytes by index returns an integer:

  In [1]: b"foo\n"[-1]
  Out[1]: 10

This means the condition is accidentally always true. Fix that by using
endswith().

Found by Claude Opus 4.8.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/util/helper.py

@@ -135,7 +135,7 @@ def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=
         if buf:
             # process incomplete line
             res.append(buf)
-            if buf[-1] != b'\n':
+            if not buf.endswith(b'\n'):
                 buf += b'\n'
             for callback in self.callbacks:
                 callback(buf, process)