nixos/test-driver: handle decoding errors in Machine.execute

The output of a command is not guaranteed to be valid UTF-8, so the
decoding can fail raising UnicodeDecodeError. If this happens during a
`succeeds` the check will be erroneously marked failed.

This changes the error handling to the "replace" mode, where invalid
codepoints are replaced with � (REPLACEMENT CHARACTER U+FFFD) and the
decoding can go on.

nixos/lib/test-driver/test_driver/machine.py

@@ -545,7 +545,7 @@ class Machine:
         self.shell.send("echo ${PIPESTATUS[0]}\n".encode())
         rc = int(self._next_newline_closed_block_from_shell().strip())
 
-        return (rc, output.decode())
+        return (rc, output.decode(errors="replace"))
 
     def shell_interact(self, address: Optional[str] = None) -> None:
         """Allows you to interact with the guest shell for debugging purposes.